Windows likes to play with drive letter assignments, if you've plugged in any other U.S.B. devices like a phone, or p.d.a. Windows may have forgotten what the hard drive's letter assignment was. Try this:

1. Log on as Administrator or as a member of the Administrators group.
2. Click Start, click Control Panel, and then click Performance and Maintenance.
   
   Note If you do not see Performance and Maintenance, go to step 3. Performance and Maintenance appears in Control Panel only if you use Category view. If you use Classic view, Performance and Maintenance does not appear.
3. Click Administrative Tools, double-click Computer Management, and then click Disk Management in the left pane.
4. Right-click the drive, the partition, the logical drive, or the volume that you want to assign a drive letter to, and then click Change Drive Letter and Paths.
5. Click Add.
6. Click Assign the following drive letter if it is not already selected, and then either accept the default drive letter or click the drive letter that you want to use.
7. Click OK.