Macintosh HDD Maintenance

Apple (Quantum) 40MB HDD

The internal hard drives in vintage Macintosh computers are SCSI drives, most of them made for Apple by Quantum and Conner and carrying Apple labels. After thirty-plus years the common faults are heads stuck by decaying rubber bumpers, stale drivers, SCSI ID and termination mistakes, and a tired power supply. Copy the data off any drive that still works before experimenting with it.

Drive spins up, then stops

Sticky head-stop bumpers

HDD Head Rubber Bumpers

Many Quantum ProDrive models (LPS and ELS, 40 MB to 500 MB) fail the same way. The drive spins up, the usual pattern of start-up clicks does not come, and after a few seconds it spins down again. The head stack is held by one of two rubber bumpers at the ends of its travel. The one it parks against has turned to a sticky goo, and the voice coil is not strong enough to pull the head off it, so the firmware gives up.

On the Quantum drives the troublesome bumper is the inner one, under the platter, and cannot be replaced without removing the platter. The outer bumper can be reached by lifting the top magnet on older models. Quantum's AIRLOCK latch holds the heads near the spindle until the platters are spinning.

Fixes, as reported by Doug Brown:

• To recover data: with the cover off and the platters at full speed (latch released), move the head gently off the bumper by hand. The drive then runs its start-up seek test and can be read.
• For a longer-term repair: a plastic insert sold on eBay as the "Quantum ProDrive ELS Repair Insert" stops the head reaching the bumper.
• Conner drives such as the CP30175E stick in a similar way and were more sensitive to timing; a strip of Kapton tape that stops the head reaching its parked position made one work, with the caveat that the head may then rest over a data area when powered off.

Brown found these older drives tolerated having the cover removed for the repair, unlike modern high-density drives.

Drive spins but does not mount

Apple's first step "before you replace any drive hardware" is to run HD SC Setup to update the driver on the drive; HD SC Setup works with both SCSI and IDE drives. Then check the SCSI ID and termination: every device on the chain needs its own ID, and the chain must be terminated. The machine's own Service Source symptom chart gives the order of checks; the Color Classic chart, for example, starts with removing any SCSI loopback card, checking that the internal drive has its three terminating resistor packs, and checking external termination.

SCSI ID jumpers on Apple's replacement drives are three pairs of pins (A0-A2 or E1-E3) that set the ID in binary. Apple recommends ID 1 for a drive in a DuoDock and 2 to 6 for an external case.

Power supply

A drive that will not spin, or that stops as it spins up, can be starved by the supply. Measure the rails at the drive's power connector with the drive connected and the machine running, and check for dips during spin-up.

Apple's rail-adjustment procedure with test cable 077-0135 (red lead 4.85-5.15 V, orange lead 11.90-12.75 V) belongs to the Macintosh 128K, 512K and Plus. Its trimmer, R56 on the power/sweep board, is the supply's only voltage adjustment, so +5 V and +12 V move together when it is turned, and Apple's procedure checks both rails while setting it. Later machines have no voltage adjustment in their Service Source; the Color Classic's Adjustments chapter, for example, covers only geometry, video, convergence, geometric distortion, yoke and light-meter set-up. On those machines an out-of-specification supply is recapped or replaced, not adjusted.

Apple's procedures for replacing a drive

Terminator resistors (Quadra 900/950)

Apple's service replacement drives for the Quadra 900 and 950 must have their terminator resistors removed before fitting. Each procedure carries the caution: "Failure to remove the terminator resistors can damage the logic board."

• 160 MB drive: pull up on the tab to lift the resistors out; attach the SCSI select cable tab-up.
• 230 MB drive: pull the resistors with needle-nose pliers; attach the SCSI select cable as shown in Apple's figure.
• 400 MB drive: remove the resistors; attach the SCSI select cable with its tab facing down.

Carrier screws

Tighten the four drive-to-carrier screws in Apple's sequence to 8.0 in-lb. Apple's torque driver is preset to 8 in-lb, the sequence is the same for every drive and carrier, and Apple warns that a drive not properly torqued to its carrier can be damaged.

Revision matching

The Hard Drive 20SC 3.5-inch Revision A (661-0373) and Revision B (661-0612) must be replaced like-for-like. Rev A replacements ship in their internal carrier component side up, Rev B component side down.

External cases

To fit a replacement mechanism into an external case, remove its three terminator resistor packs, connect the case's SCSI select cable to the six select pins and the case's LED cable. If Get Info then shows the wrong SCSI ID or drive name, reverse the select-cable connector.

Further reading on this wiki

• Macintosh II Repair and Upgrade Secrets: chapter 9, SCSI hard drive upgrades (p. 205): bus ID, pin header locations and labelling, termination, terminator orientation and terminator filters.
• Troubleshooting Your Macintosh 1992: chapter 10, SCSI and hard disk drives (p. 231).

Related pages

• Hard Drive Maintenance and Repair
• Macintosh Floppy Drive Maintenance
• Macintosh SE/30 Troubleshooting
• Macintosh SE General Maintenance
• Apple Service Source