The '''Apple Hard Disk 20 (HD20)''' was Apple’s first external hard drive designed for early Macintosh models. It features a 20 MB Rodime RO552 mechanism housed in an external enclosure. Unlike later SCSI drives, the HD20 uniquely connects through the Macintosh floppy disk port, utilizing a proprietary interface. Due to aging components, HD20 units frequently require thorough maintenance and careful restoration to remain operational.
The '''Apple Hard Disk 20 (HD20)''' was Apple's first hard drive for the Macintosh. It does not use SCSI: it connects to the external floppy disk drive port and speaks Apple's '''Directly Connected Disk (DCD)''' protocol, which is why it works with a Macintosh 512K and why it cannot be moved onto a SCSI chain.
== Disassembly Instructions ==
The mechanism inside is a '''Rodime RO552''', a 3.5-inch drive. The photograph of the mechanism on this page was taken from a unit in this collection.
''Warning:'' Always unplug the HD20 and use anti-static precautions before disassembly. The plastic case may have become brittle; handle with care.
=== Tools Required ===
This page follows Apple's own ''[[Apple Hard Disk 20 Service Source]]'' for the disassembly order and its cautions. Where Apple published no figure - service intervals, storage humidity, lubrication schedules - this page says so rather than offering one.
* Torx screwdriver (bottom screw)
* Small flat-head screwdriver or plastic pry tool (for case tabs)
== How it connects, and why that matters ==
* Phillips screwdriver (internal screws)
The DCD interface uses the DB-19 disk port, but almost none of it the way a floppy drive does. Apple's specification assigns PH0 to PH2 to handshaking, PH3 to selecting between multiple DCD devices, IENBL2 as the enable, RD as read data and sense, and WR as write data.<ref name="dcd">''Rigid Disk Interface Specification / Software Protocol for Directly Connected Disks'', version 1.2a, Apple Computer, Inc., May 1985 - hosted on this wiki as [[:File:Apple Directly Connected Disks Specification 1985.pdf]]. The DB-19 pinout as used by a DCD device, with PH0-PH2 for handshake, PH3 for multiple devices, IENBL2 as enable, RD as read data and sense and WR as write data; the marking of pins 5 to 8 as "N/C - DCD to be self-powered"; serial transmission at 489.58 kbit/s (2.042 microsecond data cell) for a net 428.38 kbit/s, set by the Macintosh's 7.8333 MHz clock; the seven-data-bytes-to-eight-transmitted-bytes grouping required by the IWM's always-set MSB.</ref>
Two consequences for anyone working on one:
* '''The HD20 is self-powered.''' The −12 V, +5 V and +12 V pins that the port offers a floppy drive are marked "N/C ... DCD to be self-powered".<ref name="dcd" /> The computer supplies no power to the drive, so a Macintosh that will not start with the HD20 connected has a signal problem or a cable problem, not a current problem.
* The link runs at '''489.58 kbit/s''', a rate fixed by the Macintosh's 7.8333 MHz clock rather than a round number, for a net data rate of 428.38 kbit/s after the IWM's most-significant-bit overhead.<ref name="dcd" />
== Disassembly ==
''Warning:'' unplug the HD20 before opening it. The plastic case becomes brittle with age.
Apple's order, from the Service Source, is '''cover, controller board, bottom shield, fan, power supply, hard drive''' - each step depends on the one before.<ref name="hd20ss">''Apple Hard Disk 20 Service Source'' (Take Apart and Exploded View), Apple Computer, Inc. - hosted on this wiki at [[Apple Hard Disk 20 Service Source]], file [[:File:Hard disk 20.pdf]]. Cover removal: one Torx screw in the case bottom, a jeweller's screwdriver to release the tabs, the caution "Do not use a screwdriver to separate the case" and the instruction to use the Macintosh pull-apart tool. Controller board: remove the grounding screw, disconnect the power supply, hard disk, fan and interface cables, remove four mounting screws. Bottom shield: two retaining screws, lifted out with the power supply, hard drive and fan still attached. Fan: two mounting screws, with the replacement note that the air flow arrow points to the rear of the case and the rotation arrow points down. Power supply: three mounting screws. Hard drive: disconnect the drive cable and remove four mounting screws, with the replacement note that "the hard drive cable is not a standard ribbon cable. An incorrect cable will cause serious damage to the hard drive and/or controller board."</ref>
=== Tools ===
* Torx screwdriver for the single case screw
* Jeweller's screwdriver to release the case tabs
* A Macintosh pull-apart tool, or a plastic case opener
* Phillips screwdriver for the internal screws
* Anti-static wrist strap
* Anti-static wrist strap
=== Step-by-Step Disassembly ===
=== Procedure ===
# [[File:HD20 Inside.jpg|thumb|Apple Hard Disk 20 (Lid Removed)]]'''Remove bottom screw:''' Turn unit upside down and remove the Torx screw securing the case.
# [[File:HD20 Inside.jpg|thumb|Apple Hard Disk 20 with the lid removed]]'''Cover.''' Turn the unit upside down and remove the single Torx screw from the case bottom. Use a jeweller's screwdriver to '''release''' the tabs that hold the cover. Apple's caution is explicit: '''"Do not use a screwdriver to separate the case."''' Separate it with a Macintosh pull-apart tool or a plastic case opener.<ref name="hd20ss" />
# '''Release case tabs:''' Carefully use a flat-head or plastic tool to gently disengage the plastic tabs along the case seam without cracking them.
# '''Controller board.''' Remove the grounding screw. Disconnect four cables - power supply, hard disk, fan and interface. Remove the four mounting screws and lift the board out.<ref name="hd20ss" />
# '''Lift the top cover:''' Once tabs release, gently lift off the cover.
# '''Bottom shield.''' Remove the two retaining screws. The shield lifts out with the power supply, hard drive and fan still attached to it.<ref name="hd20ss" />
# '''Disconnect internal cables:''' Unplug the power supply, hard disk ribbon cable, fan cable, external interface cable, and LED indicator (if present) from the controller board.
# '''Fan.''' Two mounting screws. On reassembly, '''the air flow arrow points to the rear of the case and the rotation arrow points down.'''<ref name="hd20ss" />
# '''Remove the controller board:''' Unscrew the grounding screw and four corner screws, then remove the controller board carefully.
# '''Power supply.''' Three mounting screws.<ref name="hd20ss" />
# '''Remove RF shielding:''' If present, remove screws holding the metal RF shield and lift it out.
# '''Hard drive.''' Disconnect the drive cable, remove the four mounting screws and lift the drive out.<ref name="hd20ss" />
# '''Remove the cooling fan:''' Unscrew fan mounting screws, lift out, and disconnect remaining wires.
# '''Remove power supply (PSU):''' Unscrew mounting screws, lift out, and disconnect PSU cables.
=== The cable that will destroy the drive ===
# '''Remove hard drive:''' Disconnect any remaining cables and remove the four screws securing the Rodime drive. Lift the drive out gently.
Apple's replacement note on the hard drive step is the most important sentence in the whole document:
<blockquote>The hard drive cable is not a standard ribbon cable. An incorrect cable will cause serious damage to the hard drive and/or controller board.<ref name="hd20ss" /></blockquote>
== Cleaning and Preservation ==
Do not substitute a generic IDC ribbon cable because the connectors fit. If the original cable is damaged, trace it out before making a replacement.
Proper cleaning ensures longevity and functionality.
=== External Cleaning ===
== Cleaning and preservation ==
Clean the enclosure with mild detergent or isopropyl alcohol. Avoid harsh solvents. Consider retrobrite treatment carefully if yellowed.
=== Internal Electronics Cleaning ===
=== External ===
Use compressed air and brushes to remove dust from electronics. Clean corrosion or sticky residue gently with isopropyl alcohol and cotton swabs.
Mild detergent or isopropyl alcohol on a cloth. Avoid solvents. See [[Retrobrite]] before committing a yellowed case to peroxide treatment - the results are not permanent.
=== Rodime Drive Cleaning ===
=== Internal electronics ===
Avoid opening the drive unless necessary. Clean external contacts gently. Internal cleaning should only be performed in dust-free conditions if head stiction occurs, using 99% isopropyl alcohol and lint-free swabs.
Compressed air and a soft brush for dust; isopropyl alcohol and cotton swabs for residue and light corrosion.
=== Preventative Preservation ===
=== The mechanism ===
Consider replacing electrolytic capacitors in both the power supply and Rodime logic board. Replace degraded rubber components. Store in a low-humidity, stable environment.
[[File:RodimeRO552.jpg|thumb|Rodime RO552 with the lid off]]
'''Do not open the head-disk assembly.''' The photograph here shows one opened for documentation, not as a repair step. Opening a sealed HDA outside a clean environment contaminates the platters, and a contaminated platter takes the heads with it.
== Troubleshooting and Diagnostics ==
'''Do not oil the spindle bearing.''' Advice to put "two or three drops of light oil" into an HDA circulates widely and has no manufacturer basis. Oil that reaches a platter surface ends the drive; a spindle that has actually seized is a mechanical failure, and the realistic options are a donor drive or an emulator. If a drive will not spin up, check the supply and the drive electronics first - a seized spindle is much rarer than a failed supply.
Use systematic steps to diagnose issues:
* '''No power/spin-up:''' Check power supply voltages (+5V and +12V). Inspect for blown fuse or failed capacitors.
=== Capacitors ===
* '''Spins but not recognized:''' Confirm cable connections; check head stiction or faulty controller board components.
Replacing the electrolytics in the power supply and on the drive's own logic board is reasonable preventive work on a unit of this age. See [[Capacitor Failure Symptoms]] for what failing ones look like and [[Hard Drive Maintenance and Repair]] for general drive-side work. This wiki has not found an Apple component listing for the HD20 supply, so no values are given here; read them off the parts fitted.
* '''Drive spins down immediately:''' May indicate mechanical seizure or PSU overload.
* '''Unusual noises:''' Grinding (dry bearings), repeated clicking (head seek issues), no noise but heat (stalled motor).
* '''Data errors:''' Check for bad sectors or corrupted disk format. Use Apple’s HD20 Test Utility for diagnostics.
== 🔧 Component-Level Repairs ==
== Troubleshooting ==
Detailed repairs ensure reliability:
* '''No power, no spin-up.''' Check the mains lead and the fuse, then the power supply outputs at the connector against the markings on the supply itself. The drive draws nothing from the Macintosh, so the computer is not the variable here.<ref name="dcd" />
* '''Spins but is not recognised.''' Check the interface cable and the controller board connections. The HD20 needs the Macintosh to have HD20 support - either in ROM or loaded from the startup disk - so a machine that has booted from a disk without it will not see the drive. This is a software condition, not a fault.
* '''Spins down immediately.''' Either the supply is collapsing under load or the mechanism is drawing too much. Disconnect the drive and see whether the supply holds up on its own.
* '''Noises.''' Repeated clicking is the heads failing to find servo information; grinding is mechanical and means stop immediately, because continuing costs you the platters.
* '''Data errors.''' Apple's own '''HD-20 Tests''' utility exists and is archived; run it before concluding the mechanism is failing.
=== Spindle Motor Lubrication ===
== Component-level repairs ==
Address spindle motor freezing due to dried lubricant:
# Gently rotate spindle manually; power briefly to test spin-up.
=== Head Stiction Repair ===
=== Controller board ===
Resolve heads sticking to platters:
[[File:HD20 Control Board.jpg|thumb|Apple HD20 controller board]]
# Open drive in a clean environment.
Inspect for cracked solder joints, particularly around the connectors that take mechanical strain, and for corrosion. Reflow suspect joints. Remember that the board and the drive are both destroyed by the wrong drive cable, so check the cable before powering up a board you have worked on.<ref name="hd20ss" />
# Carefully detach stuck heads using a thin plastic tool.
# Clean heads/platter with alcohol; remove sticky bumpers if present.
=== Power Supply Repairs ===
=== Power supply ===
Recap aging PSU capacitors. Check fuse, transistors, and voltage regulators. Replace with identical or higher-spec components.
Recap, check the fuse, and check the semiconductors on the primary side. Mains potential is present inside this enclosure when it is plugged in.
=== Controller Board Repairs ===
=== Fan ===
[[File:HD20 Control Board.jpg|thumb|Apple HD20 Control Board]]
Clean it, and re-oil the sleeve bearing if it is that type. On refitting, get the orientation right: air flow arrow to the rear, rotation arrow down.<ref name="hd20ss" />
Inspect for cracked solder joints and damaged ICs. Reflow solder connections and replace visibly damaged components.
=== Cooling Fan Maintenance ===
== Storage and returning to service ==
Clean and lubricate the fan, or replace if faulty to ensure adequate cooling.
Store the unit somewhere dry and stable, out of direct sunlight, and inspect before applying power after a long period: look for corrosion, rodent damage and anything that has grown in the case.
== Known Mechanical Failures ==
'''No manufacturer figures exist for HD20 storage conditions, and Apple published no maintenance schedule for it.''' Specific temperature and humidity bands, and advice to spin the drive up periodically "to redistribute the lubricant", are not Apple recommendations. Running a drive occasionally does let a fault announce itself while you are watching, which is worth something; it does not preserve anything.
Common mechanical issues include dried spindle bearings, head stiction, degraded rubber bumpers, spindle motor failures, head crashes, worn stepper mechanisms, and cooling fan failure. Regular inspection and lubrication mitigate these issues.
== Long-Term Storage and Power-Up ==
'''Back up the contents the first time the drive comes back to life''', before doing anything else with it. A drive of this age that reads today may not read next week.
Store in a climate-controlled environment (10–27 °C, 40–50% humidity). Spin up the drive periodically to maintain lubricant distribution. Inspect thoroughly and lubricate before powering up after long-term storage. Always back up data immediately after reviving the drive.
The Apple Hard Disk 20 (HD20) was Apple's first hard drive for the Macintosh. It does not use SCSI: it connects to the external floppy disk drive port and speaks Apple's Directly Connected Disk (DCD) protocol, which is why it works with a Macintosh 512K and why it cannot be moved onto a SCSI chain.
The mechanism inside is a Rodime RO552, a 3.5-inch drive. The photograph of the mechanism on this page was taken from a unit in this collection.
This page follows Apple's own Apple Hard Disk 20 Service Source for the disassembly order and its cautions. Where Apple published no figure - service intervals, storage humidity, lubrication schedules - this page says so rather than offering one.
How it connects, and why that matters
The DCD interface uses the DB-19 disk port, but almost none of it the way a floppy drive does. Apple's specification assigns PH0 to PH2 to handshaking, PH3 to selecting between multiple DCD devices, IENBL2 as the enable, RD as read data and sense, and WR as write data.[1]
Two consequences for anyone working on one:
The HD20 is self-powered. The −12 V, +5 V and +12 V pins that the port offers a floppy drive are marked "N/C ... DCD to be self-powered".[1] The computer supplies no power to the drive, so a Macintosh that will not start with the HD20 connected has a signal problem or a cable problem, not a current problem.
The link runs at 489.58 kbit/s, a rate fixed by the Macintosh's 7.8333 MHz clock rather than a round number, for a net data rate of 428.38 kbit/s after the IWM's most-significant-bit overhead.[1]
Disassembly
Warning: unplug the HD20 before opening it. The plastic case becomes brittle with age.
Apple's order, from the Service Source, is cover, controller board, bottom shield, fan, power supply, hard drive - each step depends on the one before.[2]
Tools
Torx screwdriver for the single case screw
Jeweller's screwdriver to release the case tabs
A Macintosh pull-apart tool, or a plastic case opener
Phillips screwdriver for the internal screws
Anti-static wrist strap
Procedure
Apple Hard Disk 20 with the lid removedCover. Turn the unit upside down and remove the single Torx screw from the case bottom. Use a jeweller's screwdriver to release the tabs that hold the cover. Apple's caution is explicit: "Do not use a screwdriver to separate the case." Separate it with a Macintosh pull-apart tool or a plastic case opener.[2]
Controller board. Remove the grounding screw. Disconnect four cables - power supply, hard disk, fan and interface. Remove the four mounting screws and lift the board out.[2]
Bottom shield. Remove the two retaining screws. The shield lifts out with the power supply, hard drive and fan still attached to it.[2]
Fan. Two mounting screws. On reassembly, the air flow arrow points to the rear of the case and the rotation arrow points down.[2]
Hard drive. Disconnect the drive cable, remove the four mounting screws and lift the drive out.[2]
The cable that will destroy the drive
Apple's replacement note on the hard drive step is the most important sentence in the whole document:
The hard drive cable is not a standard ribbon cable. An incorrect cable will cause serious damage to the hard drive and/or controller board.[2]
Do not substitute a generic IDC ribbon cable because the connectors fit. If the original cable is damaged, trace it out before making a replacement.
Cleaning and preservation
External
Mild detergent or isopropyl alcohol on a cloth. Avoid solvents. See Retrobrite before committing a yellowed case to peroxide treatment - the results are not permanent.
Internal electronics
Compressed air and a soft brush for dust; isopropyl alcohol and cotton swabs for residue and light corrosion.
The mechanism
Rodime RO552 with the lid off
Do not open the head-disk assembly. The photograph here shows one opened for documentation, not as a repair step. Opening a sealed HDA outside a clean environment contaminates the platters, and a contaminated platter takes the heads with it.
Do not oil the spindle bearing. Advice to put "two or three drops of light oil" into an HDA circulates widely and has no manufacturer basis. Oil that reaches a platter surface ends the drive; a spindle that has actually seized is a mechanical failure, and the realistic options are a donor drive or an emulator. If a drive will not spin up, check the supply and the drive electronics first - a seized spindle is much rarer than a failed supply.
Capacitors
Replacing the electrolytics in the power supply and on the drive's own logic board is reasonable preventive work on a unit of this age. See Capacitor Failure Symptoms for what failing ones look like and Hard Drive Maintenance and Repair for general drive-side work. This wiki has not found an Apple component listing for the HD20 supply, so no values are given here; read them off the parts fitted.
Troubleshooting
No power, no spin-up. Check the mains lead and the fuse, then the power supply outputs at the connector against the markings on the supply itself. The drive draws nothing from the Macintosh, so the computer is not the variable here.[1]
Spins but is not recognised. Check the interface cable and the controller board connections. The HD20 needs the Macintosh to have HD20 support - either in ROM or loaded from the startup disk - so a machine that has booted from a disk without it will not see the drive. This is a software condition, not a fault.
Spins down immediately. Either the supply is collapsing under load or the mechanism is drawing too much. Disconnect the drive and see whether the supply holds up on its own.
Noises. Repeated clicking is the heads failing to find servo information; grinding is mechanical and means stop immediately, because continuing costs you the platters.
Data errors. Apple's own HD-20 Tests utility exists and is archived; run it before concluding the mechanism is failing.
Component-level repairs
Controller board
Apple HD20 controller board
Inspect for cracked solder joints, particularly around the connectors that take mechanical strain, and for corrosion. Reflow suspect joints. Remember that the board and the drive are both destroyed by the wrong drive cable, so check the cable before powering up a board you have worked on.[2]
Power supply
Recap, check the fuse, and check the semiconductors on the primary side. Mains potential is present inside this enclosure when it is plugged in.
Fan
Clean it, and re-oil the sleeve bearing if it is that type. On refitting, get the orientation right: air flow arrow to the rear, rotation arrow down.[2]
Storage and returning to service
Store the unit somewhere dry and stable, out of direct sunlight, and inspect before applying power after a long period: look for corrosion, rodent damage and anything that has grown in the case.
No manufacturer figures exist for HD20 storage conditions, and Apple published no maintenance schedule for it. Specific temperature and humidity bands, and advice to spin the drive up periodically "to redistribute the lubricant", are not Apple recommendations. Running a drive occasionally does let a fault announce itself while you are watching, which is worth something; it does not preserve anything.
Back up the contents the first time the drive comes back to life, before doing anything else with it. A drive of this age that reads today may not read next week.
↑ 1.01.11.21.3Rigid Disk Interface Specification / Software Protocol for Directly Connected Disks, version 1.2a, Apple Computer, Inc., May 1985 - hosted on this wiki as File:Apple Directly Connected Disks Specification 1985.pdf. The DB-19 pinout as used by a DCD device, with PH0-PH2 for handshake, PH3 for multiple devices, IENBL2 as enable, RD as read data and sense and WR as write data; the marking of pins 5 to 8 as "N/C - DCD to be self-powered"; serial transmission at 489.58 kbit/s (2.042 microsecond data cell) for a net 428.38 kbit/s, set by the Macintosh's 7.8333 MHz clock; the seven-data-bytes-to-eight-transmitted-bytes grouping required by the IWM's always-set MSB.
↑ 2.002.012.022.032.042.052.062.072.082.09Apple Hard Disk 20 Service Source (Take Apart and Exploded View), Apple Computer, Inc. - hosted on this wiki at Apple Hard Disk 20 Service Source, file File:Hard disk 20.pdf. Cover removal: one Torx screw in the case bottom, a jeweller's screwdriver to release the tabs, the caution "Do not use a screwdriver to separate the case" and the instruction to use the Macintosh pull-apart tool. Controller board: remove the grounding screw, disconnect the power supply, hard disk, fan and interface cables, remove four mounting screws. Bottom shield: two retaining screws, lifted out with the power supply, hard drive and fan still attached. Fan: two mounting screws, with the replacement note that the air flow arrow points to the rear of the case and the rotation arrow points down. Power supply: three mounting screws. Hard drive: disconnect the drive cable and remove four mounting screws, with the replacement note that "the hard drive cable is not a standard ribbon cable. An incorrect cable will cause serious damage to the hard drive and/or controller board."
