It's somehow weird that a USB drive mounts as A. As far as I know, the A and B letters are reserved for floppy drives. However, it's easier to change its letter. Therefore, go to Start and type Computer Management. Locate the Disk management section in the tree menu. Right click the USB drive and select Change drive letter and paths. Pick another letter. Accept the changes. Replug the USB drive and then you'll have the possibility of using it in Desktop Media.
This is how it should look like: