I've never seen anyone post with that issue before. I think it could be an issue with the external drive. It wouldn't surprise me if loading the TM preference pane triggers a read of the TM drive.
If it were me, I'd try to eject the drive, and unplug it from the Mac. Then open the TM preference pane and see if it opens OK. If it does open OK, it wouldn't appear to be a software issue so I would suspect the external drive even more.
You could try to use Disk Utility to do "first aid" on the external.
Is the HDD very old?