Sounds to me like a termination problem.
Lack of proper termination is probably the most likely problem but other possible problems:
lack of term power (not likely with old SCSI drives but check the jumpers)
SCSI "proto" chip
burnt out SCSI diodes (common on early 3000 motherboards, often visible)
New Rev 7 Ramsey + DMAC 4 can cause problems with some accelerators and AmigaOS 3.9
