My laptop running Windows 8.1 couldn't see either of my external USB hard drives after they were disconnected/reconnected later on. The hard drives were not failing and were able to be accessed through another laptop. I finally came to the conclusion these hard drives were improperly unmounted, which caused Windows to get confused about their drive letters.
I was able to restore access after deleting the drive letters for both external drives under this registry entry HKLM\SYSTEM\MountedDevices. I've reconfigured the drives to be unmounted through the safely remove hardware and eject media applet to see if it solves the problem.
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