I have a StorageTek L700 with 6 drives. The drives are controlled by 3 dual
channel adaptec SCSI cards.

When I run duplications I would like to organize it so that it reads from one
tape drive and writes to another tape drive on the same SCSI card.

Card1 has Drive0 and Drive1
Card2 has drive2 and drive3
Card3 has drive4 and drive5

I think it would be most efficient if I could read from drive0 and write to
drive1, read from drive2 and write to drive3, etc.
As opposed to reading from drive4 and writing to drive0 and reading from drive1
and writing to drive5. Lots of overhead on the cards and the bus!

So... I think I need to create storage units that house two drives each but I
don't know how to specify which drives belong to which storage unit.

Any recommendations would be appreciated.