I have had this problem.
Surprisingly, the answer is fairly simple.
Hard drive (a WD MyBook 500GB, and WD MyBook 1 TB, in my case) is not recognized by the computer (Windows XP Windows Explorer program). Happens intermittently, especially after my "cleaning ladies" have visited. Problem is not solved by power cycling the computer, or just rebooting the computer. Problem was solved by disconnecting the power for a few minutes from both the computer AND the disks, then restarting the whole system after restoring power.
Cables to the disk (in my case, going to an external USB 2.0 hub) were checked and found to be OK.
These WD drives have a--hard to see-- button on the back that resets their "state". Press it for a while till the disk clearly is "going into a reset or power down state"; then press it for a second or so so the disk repowers itself.
The disks have somehow become confused (I surmise because the inrush current of the cleaners vacuum cleaner has disturbed the state machine in the disks). Rebooting the computer does nothing to fix the disk problem, since it is the disk controller that is confused. The disk controller will eventually reset itself if the power is left off for a while(i.e., the disk is unplugged), but this is inconvenient. WD's documentation doesn't mention what the button on the back of the disks is for, but its most useful function is to power cycle the disks, and RESET THE INTERNAL STATE MACHINE.