I have four or five externals laying around the house and two at work. I also have an old MAC Pro that is going on 12 years now with three hard drives I use for storage. It hasn't failed me yet.
External storage is cheap and it is slow with even the newest USB. If storage and backup is the goal the USB I find to be ok. I bought a a 4tb external not long ago and loaded everything on it. They are little towers and I knocked it over on its side and it died. If you go that route be sure and get two and duplicate.
The real question is do you want to work with those files or are you just storing. Externals are just to slow to work from. An internal may be quicker.
But I know the MAC Pro at its age it would be very slow if working between two internal drives.
My ASUS ROG I bought last year is very seamless between the two internal drives. Using a USB it still is to slow and I use an external for additional backup.

From my perspective I do not like your Nephews advice. I would be looking to upgrade, after all you are on year 9. You were probably storing and manipulating files from the 7D or the 5D II in 2011. For now get an external, when you upgrade you can use the external to put everything on the new machine.