If a Non-Apple firmware based disk drive or a MKXX06GAX Type drive is placed inside an Apple iPod first (or possibly third generation) the unit will repeatedly lock and unlock the heads.
Although the unit will eventually mount they are effectively unusable in this state - a Toshiba MKXX04GAL (where XX represents the size in gigabytes) variant must be used and an Apple firmware updater from the utilities folder from 05/06 used to update the replacement drive to a useable state.
A Toshiba MK4004GAL Will work, however it is not physically able to be fitted to the device. This means that only a 5GB, 10GB, 15GB, 20 and 30GB drive of a GAL variant will physically fit and be useable.