Thanks everyone, because I'm exceedingly cheap and lazy (and this is a backup solution), I ended up getting a sabrent usb dock and 4 tb HDD for now. This will also allow me to use all of the smaller drives I've been accumulating over the years. Also picked up some nice plastic drive cases to organize all of it. This is definitely the lowest tech solution, but I'm happy with it.
Until recently, I had been managing my data using large HDD's in my desktop and external usb drives. I should say that I'm mostly using this storage to backup personal data (photos/code/other data). I've gotten to a point where external drives probably aren't the best option, and I'm looking to convert an old mini-itx home-theater computer into a NAS on a tight budget. Because the PC has no 3.5" bays, I'm left with a few options:
(1) Fill the 2.5" bay with 4 tb SSDs over time (~$180-$720)
(2) Buy an appropriate ATX case and get some enterprise/NAS large spinning drives ($100 case, ~$100-$500 in drives)
(3) Use a USB 3.5" sata dock/enclosure
(4) Just buy a NAS
I realize this is small beans to this community, but I would be okay with an additional 4-12 tb of storage at this point, but realize I will want more in the future. I like option (1) based on the small footprint and simplicity, though I'm obviously overpaying for the storage. Options (2) and (4) are probably the most future proof and would have the most storage. Option (3) is likely the easiest/cheapest and could be used to expand option (1) in the future. Thanks for your thoughts.