checkk,
We use ArcserveIT at work (6.61) and had a similar problem. It all revolved around the "block size" that the Benchmark drive was configured for, if I remember correctly. We were getting very low transfer rates, but once we upped the block size, it worked great! I do not remember the exact changes we made,but if I get a chance tomorrow, I will look up the change and forward it to you.
Look at block size.....
KJ7GF