Resources

Wiki
Mac GUI – excellent description and some repair hints
User Manual
Nisha Firmware Specification
Nisha Operation Summary
Firmware, HD Diag tools, documentation

Hard Disk 20 was the first hard drive from Apple. It communicated over floppy disk drive interface and was quickly replaced by SCSI standard disks, like SC20 HDD.

Internals

Controller Board

Hard Drive

Rodime RO552 – thanks to the proprietary communication interface, this drive was manufactured for Apple only. This makes finding replacement drives extremely hard, as they don’t exist outside of full Hard Disk 20 devices.

HDD Board

Disassembled HDD Mechanics. Unfortunately the drive was in such a bad condition, that it was not possible to make it work.

Power Supply

Reassembly

Some Dirt

This drive was in a very bad condition, probably submerged in water for a long time. The electronic boards seem to be restored fine, but the disk drive could not be revived.