My Book Studio Edition II slow transfer rates

I have an odd situation that I’m hoping you can help with.  I’m using a Mac formatted My Book Studio Edition II external drive with my PC.  I connect via an eSATA connection and use the MacDrive program so my PC can read and write to the drive.  When I start transferring data between my internal hard drive and the My Book Studio external drive, I get a transfer rate of around 100 Mb/sec.  Then it starts dropping over the next few minutes, and eventually comes to rest at a crawling 5 Mb/sec transfer rate.   Any suggestions as to what’s happening?