Try the Technote #191027 on the Veritas Support page...
Seemed to cover what you are experiencing
benn dunn

Eric wrote in article
> Occasionally I get the "Drive is not responding. Backup set aborted."
> error when backing up the IS and DS of two different exchange servers.
> It only happens on occasion. Recently both exchange server IS and
> DS's failed. What's the most common cause of this?