eSATA write speed problems

When I attempt to transfer files from my internal harddisk to an attached eSATA WD My Book the transfers are incredibly slow (not sure on exact transfer rate but it took me ~20 seconds or more to transfer a 170mb file).

On the otherhand I can transfer files off the eSATA drive without any issues. The speed is great.

I've tried running a couple of HD Benchmarks and nothing seemed out of the ordinary. I've read that the JMicron JMB363 controller is rather flakey. Has anyone ever hit an issue like this and resolved it? The drive operates as expected when I hook it up USB.

What I have yet to try:
Flushing drivers from System and reinstalling them from scratch (Plan on trying this when I get home from work).

System Specs:
Motherboard: Asus P5B Deluxe / Wi-Fi
Internal HD: 150gb WD Raptor
External HD: 500gb USB/eSATA My Book