If your mobo has an unused PCIe slot, check out a RAID controller. You can get up to 10 extra SATA ports I think.
Data Hoarder
We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
Idk, maybe more computers? USB hub and sata thingies? the professional solution is a SAS card and SAS expander, which will set you back a fair bit
if you have the space, noise would be ok and cheap power: used dell server like R720 or R730 and install TrueNas
I do mini PCs and USB hard drives. More frequently accessed stuff and smaller files (think music, pictures, documents, backup repositories) resides in about 6TB of SSD storage. The only way you could go cheaper is probably raspberry pis or something but they're not nearly as good in terms of compute for the money.
Replace your low capacity drives with higher capacity drives.
The absolute cheapest way to get storage. For the barest bones, ugliest setup, I've seen people have two PC power supplies on their desk, a motherboard, a few SATA cards, and a number of drives plugged into the SATA cards, and using Linux + ZFS + Samba for the heavy lifting. Alternatively, a "NAS PC case" with a decent motherboard and such should work.
If I were building the cheapest way to have a lot of storage, but have a warranty, I'd go for a higher end QNAP NAS that supports QuTS Hero, even QES. I would then load TrueNAS SCALE on the QNAP hardware, use ZFS from there on out. This ensures a lower attack surface, and ZFS without any added stuff. The QNAP hardware isn't cheap, but it is fairly reliable.
Used HBA flashed to IT mode with a couple of breakout cables would get you 8 more sata connections. Only thing you have to watch out for is knockoffs on eBay.
You could also get a PCIe SATA expansion card for more internal storage if your case has space.