This is is usually a termination problem. That is, if you have correctly
configured your SCSI bus with the proper number of terminators (you can
refer to the SCSI Cable System manual, Apple Part #030-2042-B), the
problem probably lies with your hard disk's inability to provide
termination power to the terminator(s).
The drive doesn't work when connected to a Macintosh Plus because the
Macintosh Plus does NOT provide termination power. The same drive works
when connected to a Macintosh II or Macintosh SE -- or when another SCSI
device is connected to the bus -- because these devices supply termination
power.
Sometimes SCSI peripherals will work without proper termination --
usually only when the data lines are "clean" enough to support
communication. This cannot be relied on to continue; therefore, you
should:
- Repair the drive so that it provides termination (as it should if it
were working correctly.)
OR
- Connect another SCSI device providing termination power.
OR
- Use this hard disk on another system that provides termination power,
such as a Macintosh II or Macintosh SE.