If I remember correctly, letters A-D are supposed to reserved for drives on the system as in, "internally". This is an old coding or similar when all this came about, it helped keep the peace as it were. Which is why, in many cases, any new drive takes the next letter up that was from the line-up. In your case, this is an ext. drive and that isn't typical or you have some unique arrangement and it got hosed. I can only offer if you remove the ext. HD mounted drive inside the case and place in the PC case as 2nd or allowed connection and see what develops. Of course, SATA has it's own issues and that may refer to the reserved address for SATA 0-4 or any RAID/network arrangement if so used. You gave little details on your PC and setup, it really only a guessing game. However, not recognized on another PC, suggests that the ext. HD got labeled and thus got hosed. Since, you can't change it back other than trying the solution I offered, then you fell into a catch-22 situation. What may have happened, is you probably dismounted the ext. HD too early or some improper action and you now have the result. Have you drive mapped anything before and now a conflict arises?
-OR- if I'm following the above link actions right, you labeled the ext. HD = D, but as they provide
>>>> NOTE: It is best to select a letter between M and Z (inclusive). If you pick a letter like E, F, or G, the drive letter may still change frequently because these letters may be used for other drives, like CDROM drives, that are not always connected. <<<<
Notice the range A-D wasn't mentioned as I stated that's reserved for typical setups.
Can you take a "snipet" in Win7 of your disk management display and post on a free picture website, like Photobucket? Do your setup and then show result and snipet of display in question and post it. It helps alot to see what you really have. Post the link to your free public viewing of that pix.