Jack McCabe wants Windows to identify his data drive as D:, but D: is already taken.
We were using drive letters before DOS, and I'm surprised we're still using them. Everyone knows that C: is the main drive--or at least the one Windows boots from. (Why C:? Because A: and B: were originally reserved for floppies.) But not everyone knows that if you have an administrator-level account, you can reassign drive letters.
Here's how to change a PC's drive letters:
First, you need to get to the Disk Management tool. If you're using Windows 7, XP, or Vista, click Start, right-click Computer (in XP, right-click My Computer), and select Manage. Once the Computer Management program is up, click Disk Management. You'll find it in the left pane, under Storage.
You can't click Start in Windows 8, so press WINKEY-X and select Disk Management.
Whatever version of Windows you're using, you're now in Disk Management. You'll see a graphic list of drives and partitions on those drives. (Remember that drive letters don't actually label drives, but partitions. But if there's only one partition on the drive, it's effectively the same thing.)
Make sure that the drive containing the partition you want to change is listed. If it isn't, see My PC doesn't see the new, second hard drive for advice.
Also, check to see if another partition or device is already using the drive letter you want. If it is, you'll have to change that drive first. For instance, if you want your data partition to be D:, but your optical drive is already D:, change the optical drive to something else (say, E:, or O:). That will free up D:, allowing you to make the change you originally wanted.
I strongly suggest that you leave the boot drive (probably C:) alone.
After you click OK, a dialog box will warn you that some programs may not work. If a problem arises (and I doubt it), you can go back and change the letters again).
One last comment: You can change an external drive's letter, but the change won't be permanent. Every time you plug a storage device into a USB port, Windows will give it the first available drive letter.