Disk redundancy. Let's you plug in many drives. I have eight 16TB drives in a raid z2. I can lose some drives and not lose data.
I'd start over. A mobo, decent CPU, lots of ram for ZFS, and a used LSI storage adapter from eBay would get you far on a budget. USB won't have the performance or reliability and you'd need a handful of drives to make it worth it. You could install an M2 SATA adapter, I don't know if it would even work, and drive drives with it but they would still need power and you couldn't put the case back together without cutting holes in it.
Slackware is the solution then clearly.
I haven't used it myself, and admittedly am entirely unfamiliar, but a buddy put popos on something recently and was talking about how smooth and simple it was. Might be worth a gander.
What happens to Gus Fring on Breaking Bad. There had been all sorts of fake leaks and photoshops going around 4chan and Digg and Reddit and I sent one to my buddy, turns out it was the only real one. Whoops.