SCSI IDC 50 pin
The original HDD in PowerPC Macintosh (before G3) are SCSI drives with an IDC 50 pin connector. They are often found dead and no replacement are available (or very expensive and not necessarily in a good state).
They can be replaced by SCSI drives with an SCA-2 80 pin connector which are cheaper, faster, quieter, in better state and with higher capacity. Adapters can be found on eBay for a few bucks.
Some of these adapters come with the MTR jumper populated, it has to be removed to allow the drive to start when powering on. Other jumpers can be removed too as usually you don't need the other features and SCSI ID 0 is ok.
In theory it should also work with 68k Macs but due to software limitations only small partitions of 2GB or 4GB can be used under older Mac OS.
There are also some 2.5" drives available that could be used in PowerBooks, provided someone makes the proper adapter.