Dude, we cannot help you as long as you don't clarify what exactly you need or what you are planning.
You wanna screw in 30+ drives in some old case and hook them up to Raspberry Pies?
OK, if you think that is smart and serves your needs, do it! (Raspberry Pi is basically the worst solution when it comes to storage, it has no SATA/SAS and it has a slow NIC. Any Atom, Pentium, Celeron etc. is much better suited for that and costs only a handful more bucks)
But what was your question in the first place?
You want a storage server? - You need something with the necessary 30+ SATA/SAS ports, so either a huge mainboard or a couple of HBAs.
You want redundancy? - You need to decide weather you want hardware RAID (get HBAs with hardware RAID) or software RAID (get regular HBAs and run SnapRAID, unRAID, ZFS etc).
You want fast data transfers? - You need "2.5", "5" or "10" Gbps capable NICs and corresponding network infrastructure (switches, cables, NICs on client side). "Normal" 1 Gbps NICs. are slower than even a single modern magnetic disk drive, not even counting advantages that come combining multiple drives with technologies like RAID.
Or you just wanna hook up some old 30 GB drives and think that this would be capable of mining CHIA? Think again, I guess that wont be profitable, these Guys deal in hundreds of TB. Anyways, if this is about CHIA, check the cryptomining forum.