Sometimes when you install new hardware or remove a hard drive, the drive letters can get out of sync. For example, if you added a new hard drive on a system with an existing hard disk (C) and DVD drive (D) then the new hard drive letter would be E. If you wanted to change this letter to D and make the DVD drive E (swapping them) then you should follow the instructions in this guide.
First, load the Computer Management tool by running from the run menu (Press WINDOWS KEY + R to load this):
Then click on Disk Management on the left hand menu to bring up a selection of your current drives:
Right click on the drive that you wish to change the letter of, and select Change Drive Letter and Paths:
Click Change on this screen to select a new drive letter:
Now, choose a new drive letter for your device and click OK:
If you want to swap drive letters around, you'll have to rename one of the drives to a 3rd temporary letter to create space in the letter allocation.