I just happened upon this thread from a web search... boy, are you guys going to be happy... :-)
I bought the 750GB FreeAgent Pro a few days ago and saw the same absymal ESATA performance.
I did some research.
I found this little gem posted in a review on NewEgg's website:
This is a firmware update that will completely transform the ESATA performance!
After the update, HDSpeed is reporting ~61MB/sec read performance on my PC... still a 22ms access time, but a drastic increase in read/write performance over ESATA!
My notes on the firmware update process:
I didn't install the included JAVA VM as they directed as I already have JAVA VM V1.6.0, so installing 1.5.x was unnecessary for me.
For those of you who have never used the Oxford updater before (I've used it many times before on Oxford chip firewire enclosures).. do be careful and don't change anything you aren't sure of. Follow the instructions provided with the update and you should have no problems. Note that you may have to scan the USB bus a few times until it picks up the FreeAgent.. that's okay.
At the end of the update (and the updater says that the update was successful), the drive will just sit there glowing at you - that's normal. At this point, you need to unplug it to power it off. Wait 10 or 20 seconds... then you can power it back up and either check the esata firmware version using the oxford updater (with the drive still plugged in via usb) or unplug from usb, plug in your ESATA cable back in and reap the rewards immediately.
Also, on my machine, the drive had to be re-recognized by XP after the update (in my experience, typical behavior after an Oxford firmware update). Also, I have not tried this update on Vista, but I would think that it would work fine.