Found some great information on these boards, and I wanted to submit my specific situation for consideration. Seems like there are quite a few members (attn: fzabkar!) who are knowledgable about this specific subject. Here are the details:
My WD My Book Essential Caviar Green drive was connected to a USB hub with an incorrect, overpowered power supply. One hard drive and a mouse were fried, the other HD was miraculously spared. When I plugged the hub into my MacBook Pro Laptop, it caused the laptop to reset but it is seemingly ok, as are the two USB ports.
The My Book Essential Drive still lights and spins when plugged in, but cannot be read by any system. I cracked open the housing to discover the PCB board and the other circuit board in there showed visible signs of damage.
having the same experience as you here read up,,,,,,ok first up, some my book essential has an encryption on them...meaning if you remove your HD from the external enclosure it will be detected but the data will not or garbled. i recovered my data by using the fried pcb and used a data recover software then trow out the damage enclosure bought a new one put the HD in and reformat and recopy my data back in to it...