Destination Folder
If you change the drive letter of an external hard drive after you have created a schedule above, you will need to open up the existing schedule and edit the location of the Destination Folder.
Another words, if you assigned drive letter G as the drive letter for your external hard drive and then set up a schedule to back up files to the drive letter G, it will look at drive letter G to copy the files to. If later you change the drive letter to X, the system will still look at drive letter G to copy the files. If you do not edit the settings of the schedule, the files will not get copied.
For information on how to change drive letters, click here.