I recently bought the 500Gb Premium version from the Western Digital MyBook series of external hard disk specifically to backup my various machines using Retrospect 7. It seemed to be very flakey, however, taking ages to get going, or simply not working at all. Apparently the problem is common and may be down to Windows System Restore. You can turn this on or off on a per-drive basis:

Right-click on My Computer and choose Properties
Click the System Restore tab
Click once on the target drive and click the Settings... button
Check the box marked Turn off System Restore on this drive

Technorati tags: Western Digital MyBook external hard disk Windows System Restore Retrospect