The Apple Hard Disk 20 (HD20) is Apple's 20 MB external hard disk for the early Macintosh. It connects to the Macintosh's external floppy drive port, and up to three more hard or floppy drives can be daisy-chained from the expansion port on its controller board.[1] It does not use SCSI. It speaks Apple's Directly Connected Disk (DCD) protocol over the floppy port's signals.[2]
A Macintosh 128K or 512K needs the Hard Disk 20 Startup diskette in its internal drive to use the HD20. The Macintosh Plus does not.[1]
Apple serviced the HD20 as three exchange modules: the controller board, the power supply, and the hard disk assembly (HDA), which is the drive mechanism with its analogue board.[1] The parts list includes a "Screw (Rodime)", and the mechanism in the unit photographed below is a Rodime RO552.
Apple's documentation for the power supply stops at module level: neither the Technical Procedures nor the Service Source gives component values.[1][3]
The DCD interface uses the DB-19 disk port differently from a floppy drive. Apple's specification assigns PH0 to PH2 to handshaking, PH3 to selecting between several DCD devices, IENBL2 as the enable, RD as read data and sense, and WR as write data.[2]
The HD20 has its own power supply. 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", so the computer supplies no power to the drive.[2]
The link runs at 489.58 kbit/s, a rate set by the Macintosh's 7.8333 MHz clock, for a net data rate of 428.38 kbit/s after the IWM's most-significant-bit overhead.[2]
With both the HD20 and the Macintosh switched off, connect the interface cable to the Macintosh's external drive port and plug in the HD20's power cord.
Switch on the HD20, with the Macintosh still off. The drive "will whir and chirp", and within 15 seconds the green light on the front should be steady.
On a 128K or 512K, insert the Hard Disk 20 Startup diskette and switch the Macintosh on. On a Macintosh Plus, just switch it on.
To switch off, quit to the desktop, wait until the green light is steady, switch off the HD20, then the Macintosh. Never switch the HD20 off while the light is blinking.
The HDA is a mechanical device. Apple warns that jarring or bumping it, especially while it is running, can cause a mechanical failure or damage stored data.[1]
Warning: unplug the HD20 before opening it. Mains voltage is present in the power supply whenever it is plugged in.
Apple's tools: a Macintosh pull-apart tool, a Macintosh Torx driver, a #2 Phillips screwdriver, a jeweller's screwdriver and a medium flat-blade screwdriver.[1] The Service Source says to review ESD precautions before handling the boards.[3]
Apple Hard Disk 20 with the lid removedCover. Turn the unit upside down and remove the single Torx screw from the case bottom. Release the tab on each side with a jeweller's screwdriver and lift the cover with the pull-apart tool. Apple's caution: "Do not use a screwdriver to separate the case."[3][1]
Controller board. Remove the grounding screw. Disconnect the interface, fan, HDA and power supply cables. Remove the four mounting screws and lift the board off.[3]
Bottom shield. Remove the two screws. The shield lifts out of the plastic case with the power supply, hard drive and fan still attached.[3]
Fan. Remove the fan's screws: two in the Service Source, four in the 1985 Technical Procedures. Refit it with the embossed air-flow arrow pointing to the rear of the case. The two manuals disagree on the rotation arrow (down in the Service Source, up in the Technical Procedures), so go by the air-flow arrow.[3][1]
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.[3]
The Technical Procedures repeat it: do not substitute a different ribbon cable; replace it only with the cable in the parts list (590-0336).[1] Check this cable before powering up a controller board you have worked on.
Apple's rules before starting: test on a known-good Macintosh, then on the customer's; do not reinitialise the disk until every data-recovery step has been tried, because erasing it removes everything permanently; and if the drive never comes ready, the data cannot be recovered.[1] Back up anything readable the first time the drive works.
Check the Macintosh's external drive port with a known-good external floppy drive and the external drive port test on the MacTest diskette. If it fails, replace the Macintosh logic board.
Switch on the HD20 (not the Macintosh). You should hear it come up to speed; with the top cover off, check that the fan turns. If the drive is not running, replace the power supply, then the fan.
The LED is on when the drive is ready and flashes when the heads move. If it is not on and steady after 60 seconds, replace the controller board, then the HDA, then the power supply. Apple's note says that a light that is not on and steady after 60 seconds usually means the HDA is bad.
Run the Hard Disk Drive Diagnostic. It offers a 15-minute test and an overnight test of 20 iterations.
Whether it passes or fails, run Disk First Aid and, if the data is not already backed up, back up what you can. If it passed, reinitialise the HD20 and restore the data. If it failed, go back to the module replacements in step 3.
A corrupted System or Finder on the HD20 shows as a startup that does not complete, an ejected diskette, a lock-up, or no HD20 icon on the desktop. Apple's cures, in order, testing after each:[1]
Run Disk First Aid (on the Hard Disk Drive Diagnostic diskette) with "Repair Automatically".
Start from the Hard Disk 20 Startup diskette, holding the mouse button until the desktop appears, and copy its System and Finder onto the HD20.
Rebuild the desktop: hold Command and Option throughout startup.
Only then, initialise the HD20: hold Command, Option and Tab throughout startup. This erases the disk.
If devices chained behind an HD20 are not recognised or work intermittently, check the cables, then connect each device directly to the Macintosh. If they work directly, the fault is usually the expansion port connector on the first HD20's controller board, and Apple's cure is to replace that board.[1]
Clean dust from the boards with compressed air and a soft brush, and residue with isopropyl alcohol. For a yellowed case, see Retrobrite.
Do not open the head-disk assembly. The photograph shows one opened for documentation. Apple serviced the HDA only by exchange, and required it to be returned in Apple-approved packaging.[1]
↑ 1.001.011.021.031.041.051.061.071.081.091.101.111.121.131.141.15Apple Computer, Hard Disk 20 Technical Procedures, from Apple Service Technical Procedures, Volume Six, hosted as File:Apple Hard Disk 20 Technical Procedures.pdf. Section 1, Basics, pp. 1.2-1.3 (20 MB; external drive port; daisy-chaining; handling; switching on and off; Startup diskette needed on the 128K and 512K but not the Plus); Section 2, Diagnostics, pp. 2.2-2.3; Section 3, Troubleshooting, pp. 3.2-3.9 (three exchange modules; cable caution; flowchart and notes; startup and daisy-chaining problems); Section 4, Take-Apart, pp. 4.2-4.13 (tools; four fan screws and fan orientation, p. 4.9); Section 5, Illustrated Parts List, p. 5.3 (rev. Feb 86).
↑ 2.02.12.22.3Apple Computer, Rigid Disk Interface Specification / Software Protocol for Directly Connected Disks, version 1.2a, May 1985, hosted as File:Apple Directly Connected Disks Specification 1985.pdf. The DB-19 pinout as used by a DCD device: PH0-PH2 handshake, PH3 for multiple devices, IENBL2 enable, RD read data and sense, WR write data; pins 5 to 8 (−12 V, +5 V, +12 V, +12 V) marked "N/C - DCD to be self-powered"; serial transmission at 489.58 kbit/s (2.042 µs data cell) for a net 428.38 kbit/s, set by the Macintosh's 7.8333 MHz clock; seven data bytes sent as eight because the IWM requires the MSB set.
↑ 3.03.13.23.33.43.53.63.73.8Apple Computer, Hard Disk 20 Service Source (Take Apart and Exploded View), hosted as File:Hard disk 20.pdf, described at Apple Hard Disk 20 Service Source. Cover: 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 Macintosh pull-apart tool. Controller board: grounding screw; power supply, hard disk, fan and interface cables; four mounting screws. Bottom shield: two retaining screws, lifted out with the power supply, hard drive and fan attached. Fan: two mounting screws, air flow arrow to the rear of the case, rotation arrow down. Power supply: three mounting screws. Hard drive: drive cable and four mounting screws, with the 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."
