OK, a couple of things. First, all you have to do to move the files is drag and drop. That copies them to the external. Then you can come back after that is successful (and you test it) an then select/delete/empty trash and voila! Done!
However, that doesn't sort out a couple of things. Apps like Photoshop, Scrivener, Kindle and iTunes track where the files were the last time you opened them and won't know they've been moved unless you do something. If you decide to move ALL the music/movies/whatever in iTunes to the external, for example, do the copy of the folder (You'll find the folder to move in iTunes/Preferences/Advanced in the window labelled "iTunes Medias folder location.) and then come back to that same location in iTunes and click the Change button and enter the new location. Note that if iTunes starts and the external drive isn't there it will default to the original default location, but will show nothing there. You'll need to repoint iTunes to the location when the external reappears.
In Kindle under Preferences is a section "Content Folder" that points to where Kindle looks for the books. Again copy it, come to Kindle and change it. Test it and then delete the original folder. I don't know what Kindle will do when the external isn't there, but I suspect you'll get some error message, or an empty library.
Scrivener is a bit more tricky. I'm not sure where Scrivener puts files as a default. The files can go almost anywhere you want. I opened the Preferences in Scrivener and didn't see a default location anywhere. When I open Scrivener, the last project I had open comes up, so I suspect if I moved that to someplace Scrivener didn't know, it would complain and either make me show where it is or open a blank document. I didn't try to see which, but either would be relatively painless to sort out.
Ditto for Word. Ditto for PS.
So basically you can move the files and make the adjustments and off you go.