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.
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.