Refer the following information for the backup disk issue
How to correct slow backup performance, slow virus or pre-job scans, and agent initialization problems on heavily fragmented Windows Server Hard Disk Partitions
Information regarding the Backup Exec Backup to Disk Test Tool - B2Dtest.exe
http://support.veritas.com/docs/321584
Check the Disk Cache Settings on the B2D Target Machine. Depending on the machine's hardware and role in the network, disk caching may need to be either enabled or disabled to improve the backup performance. For more information on this, review the following Microsoft Articles:
About Cache Manager in Windows Server 2003