Win8 backup drive switched Letter, unreadable

Well if h: is still available you can try to give it back its old letter:
Start menu-> type "diskmgmt.msc" in the search field -> right click on the external drive -> Change Drive Letter and Paths -> Change... -> Select H: from the drop down menu.
Of course even if it works this is still a problem and perhaps you should report it (although I don't think that I have trouble with my external drive when it changes letters so I don't know how hard it would be to pinpoint the problem)

 

give this a try
find out what format it is (fat/fat32/ntfs ect)
copy your data somewhere safe
format the drive (in windows)

you could try partitioning it then format if the above doesnt work

or look here :
http://www.vistax64.com/general-discussion/175828-disk-management-never-starts.html

 

Have you tried any other USB external drives? There's a possibility the USB driver has become borked. Or there's the potential for some sort of malware that targets only USB drives.

Regards,
SB

 

Or the USB controller of the drive enclosure could be semi-failing. Unless if it's one of those hard drives with glued-on USB, you might still be able to salvage the drive.

vice-versa, the drive itself could be failing and have some bad clusters. Under linux, maybe loading gparted (a piece of software that looks like Windows's disk management basically) would make it crash as it scans the disk for partitions.