Windows will have a record of you using the " Drive G an external HD ". So every time you plug this into the computer, windows will recognise the drive and give you the letter "G" for it. If you were to plug it into a different usb port ( assuming it is usb ). Then windows may redetect it and may asign a different letter. Notice the two may's.
"If I were to install another Drice, would that become Drive F? "
The chances are very high, that it would.