Western Digital MyBook external drives are notorious for having a mind of their own (firmware) which spins them down seemingly every other minute (actually every 10mins). During a 60min export a friend called and asked me why it was failing repeatedly seemingly in the same location. Could the files be corrupted somehow? Asking about his set up I knew the problem when he mentioned there were two WD external drives connected to the system.
There are a few things to you can do to stop this.
01) Boot into Windows and alter their firmware with Western Digital's own tools. Good luck with this.
02) Pay $10 for No Spin. Which I haven't tried.
03) Install the FREE Keep Drive Spinning but under Snow Leopard it'll require Rosetta.
04) Download the FREE Disksomnia, which I've mentioned before, but in order to download it you must provide your email address (they email you a link to download it...evil) which opens you up to spam, their newsletters and other junk. Also, the Disksomnia installers requires your admin password which I never like. Pacifist shows that all it installs is the app and then sets it as a log-in item.
I don't like log-in items and just need the app. Yay for pacifist.
05) Mac OS X Hints has a Bash Script to keep the drives spinning. Your luck may vary with this.
This has become such a bother for me that I'm no longer using or accepting WD External drives for editing from clients.
Some suggestions I've received are essentially variations on the same idea: Put a media file on the drive and keep it playing in a loop. Small .mov, .mp3, etc...