are these MyBooks usb, firewire, or both?
I've noticed that my MyBook (firewire) cant not be seen on my Macs if I "eject" the drive until I power cycle the MyBook and unplug/replug the cable to the comps.
there is nothing physically wrong with the HDs...just a weird eject/power management issue.
imo, the amount of $$ you'll spend on buying spindles and spindles of your fav media dujour is better spent getting an extra HD and just periodically "mirroring" the drives.