Sometimes when you insert a flash memory card or plug in a USB thumb drive, Windows installs drivers for it and that’s it. The device never appears in My Computer as a drive.
The problem is Windows could not assign a drive letter for some reason. To resolve the issue, go to START -> Control Panel -> Administrative Tools -> Computer Management -> Storage -> Disk Management.
In the Disk Management window, find a disk without a letter assigned. It should be the size of the media you plugged in. Click the right mouse button on it and select Change drive letter and Paths.
Now click Add, choose a drive letter you would like for this device, and press OK.
The installed removable drive should now be accessible by that letter.