Here's the info from Apple's support page.

http://support.apple.com/kb/TA40360?viewlocale=en_US

Support of the serial HD-20s is not tied to System Software, but to the ROM
in the respective systems.

The following Macintosh models have support for the serial (non-SCSI) HD-20
built into ROM:
* Macintosh 512Ke
* Macintosh Plus
* Macintosh SE
* Macintosh Classic
* Macintosh IIci
* Macintosh Portable

The following Macintosh models do NOT have this support built in:
* Macintosh SE/30
* Macintosh II
* Macintosh IIx
* Macintosh IIcx
* Macintosh IIsi
* Macintosh IIfx
* Macintosh LC