External SATA HDD Problem

I have a Western Digitial external hard drive: My Book Premium ES - 500GB.

When I use a USB cable, I can copy a file from my C drive to MyBook at a transfer rate of 26.75 MB/s; when I use a SATA cable the transfer rate drops to 13.00 MB/s.



When I use a SATA cable connection, I can copy a file from the external drive to my C drive at 60 MB/s. Something very strange is going on.



When I use the WD diagnostic program (when using the SATA cable), sometimes it reports the drive as being IDE and at other times, it reports the drive as SATA. I think this may be when the PC is shutdown, Windows XP-SP2 may restart with a driver repair.



Thank you for any help.

I could be completely wrong as I've not had any personal experience with such drives, but I know there's a similar complaint about the Seagate eSATA drive. My guess is that USB and SATA are basically running off the same board/circuit. SATA is faster because perhaps it's having to convert more? Again, I'm not completely sure, but I'm basically trying to say it's not "true" eSATA. It's probably not a direct connection to the hard drive, assuming it is SATA.