Devices chained together with two or more 6-foot SCSI cables should be a red flag. Thou shalt use cables of good quality. Thin cables frequently introduce intermittent behavior. Thou shalt terminate the last device on the chain. The last device on a SCSI chain must have an internal or external terminator, a set of resistors designed to critically damp signals on the SCSI bus.
Termination comes in three flavors, passive, active and digital active.
Passive terminators are usually fine for all Mac systems, but active and digital active terminators don't hurt. They are more expensive. Thou shalt choose unique SCSI ID numbers for each device. Most devices have a switch, roller, or something like that which allows you to choose a SCSI ID for the device. Thou shalt not split, stub, or hot-plug.