What you have there, or should I say what you have done by changing the drive letter of the device is create a drive letter assignment clash.
For example the computer may all ready have an internal drive, and it may be partitioned.
So for example: C: windows xp os. D:backup data E: recovery os partition F: Dvd rom drive.
Now two devices, drives running in windows cannot contain the same drive letter assignment at the same time.
Drive mapping will not allow it, so it will render one drive missing from the drive list, or simply refuse to read it right. That is why you do not mess with drive letter assignments unless you know all of the ones that reside on the current system set up. when adding an external drive also.
Now you have to go into windows, click on start, then in the search box type disk man.
Click on create or format partitions.
Let the window open and look at a drive with the letter you assigned D: to right click on it.
Change the drive letter and push it to for example J or K.
Once done restart the pc, and all drives should show and be accessed without trouble or being asked to format them.
You may have to disconnect the external drive first from the computer first to get windows xp to load, but once done plug the external drive in.
It should show up as a device in disk manager, so if it does change the drive letter that way to J or K. then restart the system and all should show and work.
It is asking you to format it to assign a new drive letter since two assignments of drive letter D: exist.