From code/src wiki
SCSI2SD Support for Apple Macintosh Plus Plus
The Mac Plus is troublesome as it's bootrom code doesn't follow the SCSI standards. The SCSI standards recommend a SCSI selection abort timeout of 250ms, but the Mac Plus bootroms quit before even 1ms has expired.
- Make sure "Enable Unit Attention" is OFF
- Change SCSI Selection Delay to 0
- Enable "Respond to short SCSI selection pulses"