But first, how is it different and in what way better than just go with win 10, set up raid1 with 2 20-30TB HDD from the bios (like I have in my current build) and keep it simple.
If you pose this question on Tom's Harware forums, you'll immediately be asked why you think RAID1 is a good idea. If you say it's to keep your data safe, you'll be told RAID1 (mirroring) does not guarantee file integrity and should
not be regarded as a backup solution. I can attest to this fact because many years ago, I set up a twin 1TB hard disk bootable RAID1 Windows system. After several years, both drives started to develop bad blocks and different files on each disk were corrupted or missing. I had backups elsewhere, so nothing was lost, but it put me off RAID1 for general use.
Consider what happens if your computer is hit with a Ransomware attack. Since you're running a mirrored array, the files on both drives could end up encrypted. If you accidentally delete an important file on a mirrored array, both copies will be deleted. If your mirrored array depends on hardware and your motherboard dies, you may need to buy a similar mobo with the same RAID chipset, to restore the array (this happened to me with a 4-drive array).
Software RAID is a different matter, but trusting your data to two enormous 20 or 30TB drives in RAID1 is just asking for trouble. It's much better to keep the your 20/30TB drives in two different systems. That way, if one PC gets hit by ransomware, fire, flood, etc., with luck (and careful planning) you've still got a second copy of your data. Leaving a USB "backup" drive permanently attached to a PC will not save it from ransomware.
This True NAS sounds interesting, will do some research to see how hard it is to learn from scratch.
TrueNAS (formerly FreeNAS) is just one of many NAS/Server operating systems, many of which are free, but some cost money. There's a bewildering array of solutions and you'll go crazy trying to decide which one is "best" for you.
https://alternativeto.net/software/freenas/?license=opensource
Since you're considering 20 to 30TB disks, I assume you'll eventually end up storing 20TB to 30TB of data. As I've already said, if you stick with this capacity, keep the drives in separate systems.
https://datahoarder.io/what-is-datahoarding/
If you're determined to use RAID, consider a bunch of smaller drives set up RAID5 or RAID6 (in TrueNAS these levels are called RAID-Z1 and RAID-Z2). Very high capacity hard disks can be expensive and if a big drive fails, you'll be faced with buying a similarly priced drive if your failed drive is outside warranty. With RAID5 (RAID-Z1), you could buy five 6TB drives (total capacity 30TB) and still have 24TB usable capacity (you lose one drive's worth of capacity due to parity). In RAID6 (RAID-Z2) you'd lose two drive's capacity and end up with only 18TB for storage for five 6TB drives.
The idea behind RAID5/6 (RAID-Z1/Z2) is you can afford to lose one (or two) drives and still retain all your data. If a 6TB drive dies, it's much cheaper to replace a low capacity drive in a 30TB array, than a single 30TB drive. With (a lot of) luck, you won't lose any data due to striped parity across all five 6TB disks in the 30TB array. In practice, if a drive goes bad (corrupted files, bad blocks, dead drive) there's no guarantee you'll keep all your data during the "resilvering" process, when you replace the failed drive. If you bought all your hard disks in one batch from one supplier, when one drive fails, the others might not be far behind.
Long before I started messing around with FreeNAS/TrueNAS, I backed up all important files to multiple disks in multiple computers and external drives. Having suffered two lightning strikes near my home and lost broadband routers, hardware firewalls and network switches (but not computers) I'm aware of the fragilty of storage. As a result, I have at least 5 copies of data spread around. I keep old PCs, a Phenom 965, plus several i7-4770K systems and a 3800X, as hard disk repositories. Much easier to manage (mentally), physically separate from each other and on two different (isolated) networks. I also save large amounts of data to 800GB LTO4 tapes. Old second hand LTO4 SAS drives are dirt cheap and tapes are not too expensive either. I used 25GB BD-R optical discs in the past, but 800GB tapes are so much quicker and probably safer. Ransomware will have difficulty infecting my library of tapes (fingers crossed).
If you want to mess around with TrueNAS Core, buy a bunch of hard disks, either brand new or ex-sever pulls, plus an LSI SAS HBA controller (if you run out of mobo SATA ports), e.g LSI 9211-8i, LSI 9311-8i and try your luck with a few YouTube videos for guidance:
The beauty of TrueNAS, etc., is you don't need to shell out hundreds of dollars, pounds, euros, on a commerical NAS enclosure made by Synology or QNAP. Their cheaper offerings include slow processors (Atom) and minimal RAM (4GB). With an old PC, you've got most of the hardware apart from the drives,saving you money.
At the very least, I'd suggest 4 hard disks in RAID5/Z1/6/Z2, because you'll lose at least one drive to parity. If you can fit more drives in your case, even better. I run one 6-disk array and three 8-disk arrays. TrueNAS "loves" RAM. The more the merrier. ECC RAM is not essential, but is nice if your system supports it.
And finally, whatever you decide,
DO NOT RUN SMR DRIVES in TrueNAS.
https://www.servethehome.com/wd-red-smr-vs-cmr-tested-avoid-red-smr
https://www.truenas.com/community/resources/list-of-known-smr-drives.141/
A Fractal Define R4 case has enough room for 8 hard disks with cooling.
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