Sometimes happens to me as well so I`ve just named the volume names. In Explorer right click the drive letter and select properties. I`ve named mine for example "Freecom250" and "Backup" Not very original but I know which is which now.
Disconnecting the USB 's seems to be the main culprit Win simply finds a drive and names it in order of connection. I frequently have this happen even though my two external drives [1 FW 1 USB] are not unplugged various other devices [media cards and the like] do come and go, and Win gets the hump and shuffles the lot just show how fed up it is.
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