I have a Western Digital My Passport Studio external hard drive. It works with either Firewire 800 or USB 2.0. I've noticed that when I have it connected to Firewire, after a few hours, processes on my Mac start to go into an uninterruptible wait state. Eventually the system becomes so hard locked that I can't even shut it down. I have Spotlight indexing of the drive disabled, and the mds process seems to be the one that triggers this eventual system collapse.
This sounds suspiciously like the behaviour you see when a drive is failing.
Use Disk Utility and do a Repair on the drive, although you might need to find a different tool (such as Disk Warrior) that'll check every sector for errors.
Unless, of course, you don't see these problems when connected via USB. If USB is fine and FW is causing problems, it's probably an issue with the FW port on the drive or the computer... you'd have to get your hands on another FW device to try to figure out which.
If the problems persist via USB and/or you have problems doing a full-disk sector scan back up your data as soon as possible because the drive is dying.