Exchange Mailbox backup extremely slow - Backup Exec 9.1

Our Exchange databases have reached 200GB and are taking around 16hrs to backup using Backup Exec 9.1. The databases were recently moved to a NAS server which is reflected as an aditional drive on the Exchange box, however the problem was apparent before the move. We have a gigabit LAN and are getting anything from 400MB/min to 1000MB/min. The Exchange Mailboxes(55GB) seem to be the problem as is reflected below. They get a throughput well below 100MB/min. Is it worth doing an off-line defrag or are there any better solutions to this problem. We did an off-line defrag about a year ago. We have also tried different times of backing up and different jobs. All give the same result - slow mailbox backup. Any help would be appreciated. thanks in advance.

Set type               : BackupSet status             : CompletedSet description        : (EXCHANGE SERVER)
Resource name          : \\VIC2K07\Microsoft Exchange MailboxesLogon account          : CRPDOM\backup
Error                  : a000feab - Corrupt data encountered - See job log for details.

Agent used                     : YesAdvanced Open File Option used : YesImage used                     : No
 


Start time             : 17 January 2005 17:51:16End time               : 18 January 2005 04:51:47
Files                  : 0Directories            : 0Skipped files          : 0Corrupt files          : 11Files in use           : 0
Byte count             : 55,117,328,093 bytesRate                   : 79.00 MB/Min