Don't do that, you'll have no redundancy in case of disk errors, the performance will be abysmal (Pi4 possibly excluded) and USB drive spindown and SMART support is sketchy at best.
I bought a Fractal Node 304 case (room for 6 drives), put an ITX board in it, a PCIe SATA controller and set it up as btrfs RAID, with CIFS, NFS and FTP. Not a huge outlay and so much better than a hacked-together Pi solution.
It also functions as my DNS and DHCP (Pi-Hole in a Docker container) and since it has hardware video decoding, it works great as an always-on HTPC, which is practical for apartment living.
3 USB drives in a raid setup isn’t any less redundant than 3 SATA drives in the same setup (mathematically anyway, excluding potential bus problems which don’t really seem to be much of an issue these days).
Personally I also have a node 304 based nas, but I’ve seen plenty of people with low cost Pi setups and no major issues. Plugging a few drives into a Pi is much easier if you don’t have experience with building computers, and is still a couple hundred dollars cheaper than something like that.
Also worth noting that it’s possible to connect PCIe devices to a Pi, although I believe you need a specific model.
I bought a Fractal Node 304 case (room for 6 drives), put an ITX board in it, a PCIe SATA controller and set it up as btrfs RAID, with CIFS, NFS and FTP. Not a huge outlay and so much better than a hacked-together Pi solution.
It also functions as my DNS and DHCP (Pi-Hole in a Docker container) and since it has hardware video decoding, it works great as an always-on HTPC, which is practical for apartment living.