Transfer rate between Hard Drives
I would open up activity monitor and see what the effective instantaneous (and recent) transfer rate for data and more importantly iops.

Often many small files tax the total amount of IO operations and it's not a bandwidth issue at all. Large files that are stored sequentially on the filesystem (or when the platters and heads can not delay the transfer) will typically hit between 30 and 80 MB/s on a Mac Pro that's otherwise idle.

Similarly, more than 1,000 iops would saturate most pairs of SATA drives and perhaps far fewer IO could be limiting depending on catalog and file fragmentation or just long head seek times.