I figured out why my external USB hard drives would disappear My Computer. It was caused by a bug in a 3rd party Windows System Restore program. This issue affected computers running Windows 8/8.1 only. The author fixed this bug recently, so hopefully it doesn't happen again.
http://www.toms-world.org/blog/restore_point_creator
February 4, 2014 Version 2.2 Build 2
Fixes a major issue on Windows 8/8.1 systems in which drives were being dropped out of the My Computer listing of drives. This was due to something that was added back in version 2.1 Build 1 that had unintended side-effects. This new version includes the removal of that code as well as a fix to remove the added Registry entries that were added by the removed code. This should resolve the drive drop-outs on Windows 8.
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