Did you adjust the hard drive sleep time or the USB suspend settings or both? The suspend setting is the more important of the two.
As another troubleshooting step try a new USB cable and/or a different USB port on your computer.
As a last resort try googling your specific hard drive and the word "unmounts" or "dismounts" to make sure it's just not some inherent defect of that model.
Angelo Lorenzo
Fallen Empire - Digital Production Services