You may have a network drive that is mapped to the same letter as your internal drive. You can use Disk Management (right-click My Computer, select Manage and then Disk Management) to assign a new drive letter.
Locate the drive in the lower window and right click in the bar graph area. Right-click and select “change drive letter and path…” to change the letter to one that is not being used. (If you do not have sufficient rights, your system administrator may need to make these changes for you.)