If a new drive is missing under My Computer, the device may have a drive letter conflict and needs to be re-mapped. To correct this problem, go to My Computer and right-click to select Manage. The Computer Management panel displays lists all of the attached storage devices. Under the Disk Management menu, each storage device is shown with its assigned drive letter marked in bold. Locate the missing drive within the list and right-click on the drive name. From this menu, select "Change Drive Letter and Paths," then click on the Add button. In the Add Drive Letter or Path window, choose "Assign the following drive letter" and pick the first unused drive letter from the scroll menu.
Caution: When reassigning drive letter, be careful that the drive letter is not already being used by another attached device. Two devices cannot share the same drive letter and may cause conflict with other attached devices if assigned the same letter. After selecting the drive letter, click OK. The once invisible drive should now show up under My Computer. Close out of Computer Management.