WD My Book and eSATA data transfer rate

I'm just wondering for anyone that has used an eSATA hard drive connection, what typical speeds do you get when moving files from your computers hard drive to the external drive? I just got a Western Digital My Book Home Edition 1TB drive from Costco, and I finally got eSATA working (had some driver issues) but I'm only transferring files at a max speed of 40 MB/sec. From what I've seen on the internet, it should be much faster. I'm just wondering if I have some setting wrong or if the driver I downloaded wasn't the correct one. But I honestly don't know since this is my first eSATA drive.

I have 3 WD hdd's (one of which is a 1TB) connected by eSATA but not in WD cases I made my own using Wellend cases, they all work at the same speed as the internals which are also WD. No additional drivers were used, I simply plug it in.
This a desktop pc.