My external drive, which I use for Time Machine backups, has stopped responding 4 times in the past 2 weeks. Each time it stops responding, it also prevents the computer from shutting down normally, until I disconnect it forcefully (i.e. removing power from the drive). After rebooting the Mac and reconnecting the drive, it works again... at least for a while. The drive's a My Book, with FW 800 connectivity, and is 1TB in size.