>anybody using multiple 3590's on the same scsi controller in 3494's?

Sure, many customers...

>ANR8420E DEFINE DRIVE: An I/O error occurred while accessing drive DRIVE03.

That message, in my experience, appears when the drive is in use by
something else (tape mounted, et al).  Use a command like:
 mtlib -l /dev/lmcp0 -f /dev/rmt2 -qD
to further query the drive.  You could also check the 3494 and the drive
in it by physical inspection, and assure that the drive is marked
Available in the Library Manager.
      Richard Sims, BU