Just for a minor additional note of clarification, I was genuinely puzzled if "MFS Support" was something you were explicitly looking for. Presumably a Mac hard disk which *did* work with the 128k Mac would have its volumes formatted as MFS (as the original Hyperdrive with its "drawers" was) or otherwise would present its storage in a non-HFS manner.

(I wouldn't consider it a slam dunk that a serial-based hard disk for the Mac would actually use the Mac filesystem directly. It's quite possible a drive like that would format its disk in some proprietary way internally and behave more like a file server than a drive. Granted I have no idea how any particular unit might work without having a chance to read some documentation.)