This sometimes happens if Windows names the drive letter to something that was already in use by another device. See if you can locate the drive and change its drive letter, and this is how we do it:

1. Right click My Computer and select Manage.
2. In the right pane of the Computer Management console, go down to Storage and expand it if necessary, then click Disk Management.
3. It may take a few minutes to populate the right pane. When the disk information appears, your USB drive should show up in this list.
4. To change the drive letter, right click the drive in the list and select Change Drive Letters and Paths...
5. The currently assigned letter will be displayed in the dialog box. Click the Change button.
6. From the drop down list, pick a drive letter that is not being used by any other device and click OK. You'll get a Confirm dialog box; click Yes.