Another thing to ask here would be just how large is the external drive as well as how much actual free space is now available on it in order to see a rather large backup. If you are going with the Ultimate edition the system image for the RC would present a large problem trying to restore that on a clean install of the finished retail 7 for a start.
With multiple drives here the option of regular backups eliminates worry about losing files and running into problems later since files are stored in separate custom folders and sub folders for further separation of file types, utilities, system and hardware updates, etc. there.
For simply copying entire folders of all types from C onto the external drive I have to agree with logicearth about running into immediate problems of all sorts. Think back to how you would save files and data out of the Windows.old folder to get an idea of what you would be able to save.
The Windows Easy Transfer tool also saves settings as well as the files and folders selected. But the retail 7 will obviously be working with several changes made including the system registry itself.