Yesterday I installed a second internal hard drive in my machine.  At first I attempted to use my Windows XP CD to set up and partition the drive, but then I realized I could just do it through the DISK Manager, so I did it that way.  Both drives are working fine.  But....

I also have a Maxtor external HD HOOKED up via USB port.  The thing is chock full of files, and I've never had a single problem with it.  Now, after installing the second internal drive, my computer is treating the external drive as unformatted, unpartitioned, and empty!  I can't seem to do anything to get my machine to recognize the drive as formatted and full of data.  I tried plugging it in to my other computer, but it does the same thing, which tells me the problem is now with the external drive -- somehow something in it has been "reset" so that it's being read as a new, empty drive.  (I know I didn't accidentally format it by mistake.)

So, now I'm panicking -- I really don't want to lose the 200GB or so of data on that external drive.

Before I go hauling the Maxtor to my local shop & paying them money to fix what MAY be a very simple problem, can any of you help?  Many thanks in advance.
Would this happen to be a Maxtor, several years old? Those older ones have a problem with overheating, and failing. If so, you may want to either take it to a shop or purchase and use a recovery utility.No, it's not very old -- less than a year.  I haven't had a single problem with it since I GOT it, and it would be way too much of a coincidence that the drive would just happen to fail at the same moment I installed a new drive.  So I'm quite sure that's not the problem.Power down and unplug the external.
Do a few cold boots.
Power down and remove the 2nd internal.
Do a few cold boots.
Re-connect the external and power up...
Does this change anything ? ?I essentially tried the things you mentioned, but to no avail.

Fortunately, after trying several data recovery programs that didn't work very well, I found a good one (R-Tools) that recognizes everything on the drive and is allowing me to transfer all of the files to another drive.

Problem solved!  Many thanks for the suggestions.
Glad you got it solved...