I have a WD My Passport 650GB (with Firewire and USB). I'm using it for almost a year now and it always worked fine. While underway I simply plug it in via Firewire - at home I connect it to my Airport Extreme to have it available as a network storage.
Today I connected the HDD to my MacBook Pro (via firewire) and NOTHING. The HDD is starting (clearly making a sound and the power-indicator is flashing) but it won't appear in Finder. I also tried it with USB - no sign.
I ran Disk Utility and tried to repair the disk. At first try I got a red error line saying that something is wrong with the "headers". However the repair completed with a success message saying that everything is OK. I also verified the HDD. Also with a success message. I did that a few times again and unplugged it in between. Never got the error with the headers again - it's always completing and saying everything is OK. However I can't mount the drive.
That is what Disk Utility is showing.