My /usr/src folder occupies almost 10% of my 10GB harddrive, containing useless(?) files from kernel 2.6.20 etc. Can I move them to an external drive, and if necessary, copy them back when needed? Can I do this with just the file explorer, or is there some apt command I should run instead? Or is there a zip/deb archive of these files somewhere else that I can save instead, and delete everything in the src folder?