Is it a single disk, or a RAID array? If RAID, what level RAID? I am switching mine from RAID 5 to RAID 10 for similar reasons of poor write performance in RAID 5, is this a possibility?
Otherwise as the mod above posted, check that your AV isn't slowing it down too much.
Also are you doing it from the local machine or across your network? Obviously don't do it from the network as this will dramatically increase transfer speed/time.