To our knowledge, there is no plug-and-play hard drive for the Macintosh
512K or 128K currently still being manufactured. The Apple HD20, a non-SCSI
drive which does work with the 128K and 512K Macintoshes, was made obsolete by
the evolution of SCSI devices which brought about Apple's line of SCSI hard
drives around the beginning of 1986. SCSI drives require a SCSI port, which
is not included with a Macintosh 128, 512K or 512K Enhanced Macintosh. It
was first included with the Macintosh Plus. SCSI drives transfer data many
times quicker than non-SCSI drives.
In summary, you either need to find a used HD20 non-SCSI drive, or have a SCSI
port installed, which would be a third-party product. Dove is one company
that makes add-on SCSI ports.
WARNING: Improper installation of these kits may damage the
Macintosh logic board beyond service exchangeability.
On AppleLink, you can find contact information on a company mentioned in this
article by clicking the "Library Index" button in the lower-center of the Tech
Info Library search window, then opening the "Third Party Company Directory"
folder.