Okay, time for someone to check the drive/connector on the Mac Portable and compare it to the disk drive mechanism in the HD20. The Portable used a SCSI drive with a special cable harness attached. If the number of pins matches, it is time for a more careful comparison.I was looking at the Apple Service Source manual for the HD20 and they list the drive as part number 661-0373. This happens to also be the same part number as the Apple 20SC hard drive that shipped with the HD 20SC, the original SE 20MB configurations and may also be in the II, IIx, IIcx, IIci & IIfx, SE/30 & possibly Classic. So I checked the service source manuals for them and sure enough, Apple calls them "Revision A". Revision B is a different part number and cannot be interchanged with the Revision A drive. Unless Apple decided to reuse a part number, the drive in the HD20 appears to be a SCSI hard drive of a certain configuration.
It may be that Apple reused this modified SCSI connector in both the HD20 and the Portable, although, IIRC, the Portable had a 40MB drive.
The Portable pinout is avaiable somewhere, because there was a Portable to standard 50 pin converter available at some point. If the HD20 uses the same scheme as the Portable and we know the pinout of the Portable, then we would know the pinout of the HD20 and the only remaining thing to do would be to build a conversion cable so that we could test other SCSI drives in the HD20 case.
I'd check it myself, but I don't have a Portable.