Evidently there continues to be a known bug with RSM and the best way to
fix it is to hack the registry.  Don't know if this is what is causing
your problem, but when I talked to a veritas tech last week, this is one
of the first things he had me check when I told him that I had a 2003
media server. 

SYMPTOMS
Your Microsoft Windows Server 2003-based computer may not perform backup
jobs to tape devices on a Storage Area Network (SAN).

CAUSE
This issue may occur when a conflict exists between Plug and Play and
another process on your computer. To check the status of a backup
device, Plug and Play sends a TEST UNIT READY request to the device
every second. This request may conflict with other operations on your
computer. 

Here is a link to the technet article explaining how to do it.

http://support.microsoft.com/default.aspx?scid=kb;EN-US;842411