Andrew , are you trying to move files from F: to G ? Is so, select the files you want, right-click and select Rename, Move and Copy files. Choose rename at the top of the screen and be sure you update the Base Path in the directories section to point to the new drive letter.
If you have changed drives and all of your media is already on G:, you can use Find and Replace to update the FileName library field for all of your files.
