PROBLEM:
When an external SCSI hard disk is connected to either a Macintosh SE or
Macintosh II with an internal SCSI hard disk, neither hard disk is recognized
by the system. When the external drive is disconnected, the internal hard disk
works correctly.
Terms used in this article:
UUT = unit under test
CURE:
1) Does the system with the internal drive come up ok by itself and
do diagnostics pass ok?
Yes -- Go on to Step 2
No -- Repair machine with normal diagnostic techniques and then retest
configuration
2) Does the external SCSI device operate correctly on a Macintosh
Plus, Macintosh SE with two 800Kb drives, or on the UUT when the
internal SCSI cable is disconnected from the motherboard?
Yes -- Go on to step 3
No -- Repair external device using normal diagnostic techniques
then re-test the configuration.
3) a) Fix or replace the SCSI address select switch on the external SCSI
device
b) Fix or replace the SCSI Device select cable on the external SCSI
device
c) Check or replace the external device's SCSI terminator.
Explanation: If the drives (internal and external) work independently of each
other, chances are they conflict in one of two possible areas: Either there is
a SCSI address conflict or the SCSI buss is not terminated properly.