WD external hard drive recognition problems

I have a WD 500GB My Book external hard drive that I have been using with my laptop (Windows XP SP2) on USB since October. I recently moved back to my old apartment where I had my desktop computer set up (again Windows XP SP2). I set up the external hard drive set up on fire wire and it seemed to be working great. Then the first night I tried to shut it down and it hung. I finally manually shut off the computer and moved on. The next morning when I tried to start it, it hung on the startup screen. Figuring it must be the external HD, I shut down, unplugged it, and everything started fine. I did some reading and it sounds like you are better off stopping the external and unplugging it before you shut down anyway. No big problems there until the other day I couldn't find my external HD in My Computer. I didn't have much time but the next day when I tried again, it worked fine. This has been off and on for a couple of days. Finally today I hooked it back up to my laptop through USB and it worked great. I decided to convert the format from fat32 to ntfs on my laptop. Hooked it up to my desktop and it worked fine. I unhooked it and restarted my desktop just to make sure that cured it and of course it won't work again. I have 2 internal hard drives, each having 2 partions. I also have 2 CD drives. Someone told me that WD uses the E: drive so I even changed the letters on my CD drives to open it up and that didn't work either. I would really appreciate if anyone has any ideas. And also, when I do get it working better, am I supposed to unplug it when turning the computer on/off? I didn't have to do that on the laptop, although that was connected through USB and not firewire. Oh, I have also gone into CMOS to make sure it wasn't trying to boot off USB or Firewire. I had the CD drives then the hard drives for the boot order. Thanks again for any help.