Plug in your second USB drive. Right click on "My Computer" and select "Manage". Click "Disk Management". Right click the USB drive and select "Change drive letter and paths". Pick an unused drive letter. Whatever you pick will always be the drive letter for this USB drive.
The first one is OK. It is automatically assigned to E:, but I have quite a few network drives. My computer tries to connect to F: but that is a network drive, I removed the first drive, but the computer doesn't recognize the second one. How do I map a drive to my second USB flash drive???