Same experience here, with an external My Passport, 1 TB. Was working perfectly for a long time, containing irreplaceable data! I upgraded to Mavericks about a week or two ago, but only tried to plug it in again last weekend.
Failed to mount at all. Disk Utility can't scan it, although it DOES see it, incorrectly, as a 2.2 TB drive (?).
Following the advice on other messages on this forum, I completely eliminated any and all WD software from my machine, a Mac MBPro 2011 edition. Still, no luck, won't mount the drive, even after erasing all WD software.
I filed a support ticket, but no response at all so far.
I have another WD My Passport for Mac, which I dedicate to backups with Time Machine. Last week, it also appeared inaccessible, and I used Disk Utility to simple reinitialize the drive, since I could simply make another backup, which did work after wiping the drive.
I also downloaded and ran the Firmware update utility on that second drive, and re-scanned it to make sure, and it's fine.
The problem is with this other MyPassport, which is unreadable/unmountable. I did *not* re-run the Firmware update on it, fearing it would make matters worse, and did not re-init the drive with Disk Utility.
I'm hoping against hope WD will have a solution, and soon, to recover this data, otherwise, I may have to rip the drive out and try another enclosure from a third party. I hear that using another SATA to USB controller enclosure makes the data accessible, but would rather hear something, anything from WD.
So far... nothing.