Rich,

        Sometimes tapes will go offline if drives experience a device reset.
This could be power related or you could have faulty drives.  Marriott
experienced the same sort of issue with tapes going unavail and reado with
DLT.  Look at you error logs and see if the sense data supports those
claims.  Feel free to send it to me if you wish.  Thanks

Matt Long
Systems Administration
Marriott International