Are all the devices you are using "internal"? If not sometimes there can be issues using internal and external devices at the same time.
Is the scsi board terminated (ie with resistor packs) itself? If it is it may only "like" either all internal drives or all external drives. If the resistor packs are socketed, I suggest trying to removing them to unterminate the board itself, (ie let the SCSI signal propagate from internal to external cable). Make sure that the last devices or the end of other internal and external cables are terminated. Make sure the scsi board itself is terminated if you are using only internal or only external drives.
If the SCSI card itself has soldered on resistor packs (which I think my trumpcard pro's are), I would try all external devices, or internal devices.
