I hate to resurrect a dead thread, however I did find a cause and solution to this problem, that worked for me at least. I had a 2tb HP external drive already on my system. My problems began when I purchased 2 more (identical) drives. I found after much deliberation that all the drives were showing in Disk Management, but not in explorer. In the lower portion of Disk Management, where it shows a visual representation of the drives, there appeared a tiny speech bubble under the Disk # (ie Disk 0, Disk 1 etc) along with the word "offline". Hovering over these told me (I forget the exact wording) something to the effect that there was a a collision in digital signatures. I realized that Windows was seeing them as the same drive and attempting to assign the same letter. So using a program called Zentimo (I think there is a free version) I changed the drive letter (H) of the old hard drive to (E) and that allowed me to bring the next disk "online". I then changed this drive letter (again it was assigned as (H)) to (I), allowing the last drive to come online. I changed this drive to (J) and the original back to (H). Then I set all the drive letters as "fixed", meaning that the drives will always be assigned the same letter. Hope this helps somebody.