If the 840av is indeed the successor to/baton-carrier after the 950 it may well have both internal and external SCSI buses. While, theoretisch, the two should be independent, is it possible that your CD drives and HDDs with common SCSI IDs are conflicting? That could be the source of the failure to be visibly 'mounted' on the desktop. More so if there is but a single bus for internal and external daisy-chains. Failing that, are you the victim of termination or termination-power problems? Both contingencies are worth your looking, if only to exclude them.
de