Interesting reading about the 128K ROMs and related System SCSI problems:
http://developer.apple.com/legacy/mac/library/technotes/dv/dv_14.html
According to Apple's troubleshooting guide, if you have the Rev. 1 ROM which most 512Kes had due to the fact they didn't have a SCSI port (so it didn't matter), the SCSI drive must be powered on prior to booting the Mac, but you are probably already doing this.
However, you can eliminate this problem quickly by booting up, and press the rear interrupt switch. A dialog box will pop up with a ">" prompt. Type DM400000 and press the return key. The Hex dump in the top row, 2nd & third columns will tell you which REV ROM you have:
4D1E EExx = orig.
4D1E EAxx = Rev. 1
4D1F 81xx = Rev. 2
If you have the last one, then you can rule out the ROM involvement in the problem.
http://developer.apple.com/legacy/mac/library/technotes/dv/dv_14.html
According to Apple's troubleshooting guide, if you have the Rev. 1 ROM which most 512Kes had due to the fact they didn't have a SCSI port (so it didn't matter), the SCSI drive must be powered on prior to booting the Mac, but you are probably already doing this.
However, you can eliminate this problem quickly by booting up, and press the rear interrupt switch. A dialog box will pop up with a ">" prompt. Type DM400000 and press the return key. The Hex dump in the top row, 2nd & third columns will tell you which REV ROM you have:
4D1E EExx = orig.
4D1E EAxx = Rev. 1
4D1F 81xx = Rev. 2
If you have the last one, then you can rule out the ROM involvement in the problem.