Hi There,
When I had this issue, it was because the Backup Exec server hadn't been built with a Battery Backed Write Cache (BBWC).
Although it had a cache, without a battery, this means that the write cache will be disabled, and the write performance of the disk will be poor.
I was getting great performance to tape, and awful performance with B2D. Once the BBWC was fitted and had charged, the performance was excellent.
Does your Backup Exec server have a BBWC? If it does, does the battery need replacing?