IMPORTANT: Make sure you have the right Disk number (#)
Go to START
Go to RUN
type in cmd (command-prompt opens up)
type in chkdsk #: /R (where you should put drive letter in place of #--space before #, and after colon---example: chkdsk 1: /R)
Then it will start checking your drive, be patient here because it can take a long long time for this to finish depending on drive size. It may look in the beginning of the scan that nothing is going on, but after a few minutes it will state "THE DRIVE FORMAT IS NTFS...or FAT32 ...something like that--- and from then it starts scanning for a long time. Took me about 4 hours to check the total 750GB drive! It is checking available space and fixing any links that are present and don't belong there. This does not fix major boot records, so if you have that problem chances are you will be able to see the drive now, but still have to fix the MBR. If your purpose is to save information on the drive and you get a screen that says format the disk, say no or x out the box. In the end it should work again properly.
I don’t know how this works when you have no clue what the drive letter is, I was lucky to see this still in my disk management, but only for an instant when I unplugged the power supply while the USB was still connected...it appeared under disk drives in MY COMPUTER...but only for an instant.
If you know what it was (drive letter) before it screwed up on the computer you were using, then it is probably the same letter. However, if you improperly disconnected the drive, and a new drive letter, already assigned to something else, becomes your new drive letter you have to change back to what it was, or change the drive letter of the assigned (CD/DVD) to something else and worry about changing things back after you get the drive seen again.
If you have some interference with normal Windows functions working properly, go to Device manager (not Disk Management) and right click the drive and select disable (if this is the only operational drive, disregard this). This could be the non XP link that has it running in a loop or in direct contention with Windows programming. After running the repair (cmd), repeat the process in Device manager to update drivers, and look for new hardware if it is still not operational.
Device Manager--- "check for new hardware". You should see a screen that allows you to select the driver. In that collection, it should be the manufactures name of the drive, like Seagate blah, blah----select that and you should be back to normal.
PS--you stated your internal drive stopped working....was it labled "system" drive? It should have been. If your external is called Disk 1, then Windows has assigned your disk (0) (system to your internal drive. You can repair that drive the same way using the cmd: chkdsk 0: /R