Can an external HD be used as my primary HD?

Why can't I change my External HD to the letter it used to be?

• My external hard drive is called DANGERDOOM, and the drive letter used to be J: Then I got an identical hard drive, and the computer wouldn't read it because of a signature collision. So after some research, I discovered I could change the disk sig of my new hard drive, Liberation, through the command prompt. I unplugged DANGERDOOM when I did this, just to be sure I didn't accidentally mess it up. So after I changed the Disk sig on Liberation, I plugged both my hard drives in and now the computer will read both of them. The problem is that DANGERDOOM has gone from drive J: to drive E: which is a major problem for me. When I go into computer management, then disk management, I click change drive letter, except it won't allow me to change it back to J:, only other letters. Any suggestions?

• Answer:

  Your computer probably assigned the new drive as J: when you disconnected the old drive, right-click my computer and select "manage" then go to disc management and you will see all your drives(internal and external) from there you can change the drive letter. Change the drive letter of the new drive to something other than J: and then you should be able to change the old drive back to J: