When you say your internal HDD is giving error messages that it is failing, You should replace it with another internal HDD (probably a bigger one) after cloning the partitions to the new HDD. I think this should be your first priority. Having any number of external HDDs will only protect your data but not the functionality of a healthy laptop.
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.
This will probably help somebody, but not me. My external hdd won't show up in the disk manager at all. I've given up on it now and am looking at getting a new one to back up my hard drive while I reinstall windows (to fix windows update, possibly fixing the old external too). Thanks for the (albeit a bit late) reply anyway.
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