Hi Skub...
That's quite do-able but a little tricky in GarageBand...
You have to find the GarageBand project, right-click on it and select "show package contents" then the files you've recorded will be in the "media" folder
You can then copy them somewhere else...leaving the originals intact...
Sometimes it's best in GarageBand not to cut tracks up as it makes syncing them more awkward later on...
It's a little convoluted...I'll happily try to help if you want to PM me anytime...not that I'm an expert by any means...it's a quirky program and I know it fairly well nowadays
You can get pretty big (16/32/64gb) USB sticks for about £20...they'd do that job fine and are easy to transport...not as fast data transfer as a portable hard drive but much cheaper and more convenient, I daresay