Possible issues with defragging freespace on SAS drives?

Knowing little of the kinks behind defragmentation, I jumped headfirst into this program to see what it runs like. The initial defragmentation of a whole drive is immensely fast, and very much an improvement over the rather poor software Microsoft/Diskeeper provides us with. I'm currently testing the software on a 73,4 GB IBM Server SAS harddrive on a Promise TX2650 Raid controller (JBOD Config).

When the software begins formatting the freespace, however, it nearly stalls. The progress was stuck at 19% with little or no activity on the drive. After some time, it picked up the pace again and progressed.

Any suggestions, or am I just being paranoid?

Great piece of software, by the way :)