OK, during the pandemic, I've been putting off the day when I need to fill up my drive again and now I have 10GB free in a 42TB disk array. Yikes. Fortunately, the disk system (a Synology DS2413+) has 12 drive bays and there are two free ones. The system runs as two RAID10, so with the two free slots, so time to figure out what drives you to buy. And it appears that Synology does support large drives with a 108TB per volume limitation and on their certified drive list, they only go up to 14TB right now.
As usual, the disk drive systems continue. Normally, the highest density systems are very expensive and what is next down is half the price. I normally end up buying the highest density drives because at least at home we are more drive bay constrained than anything else so density is important.
The last time I did this, it was we ended up getting a set of 16TB drives from Seagate X16 from 2020 and got the highest end 16TB and now Seagate is up to the X20 series where we are at 20TB now and 22TB are on the way but as SMR drives (which are denser but are slow when overwriting data so you don't want random writes for drives like this, they are better for "cold" storage). In fact, the 22TB are the same platters as the 20TB but get 10% more storage by using SMR.
There are two things which are the Seagate IronWolf Pro 20TB which is tuned for small NAS systems. It uses CMR but the EXOS X20 is actually for large-scale enterprise NAS. The way to look at it is that it is rated for 300TB/year and 1.2M hours MTBF and a five-year warranty. And although it has lower specs it is usually has higher prices than its EXOS sister probably because there is more volume for enterprise drives. On Amazon it is $600
Seagate Exos X20 20TB is the sister of the Seagate IronWolf Pro that is designed just for enterprise use. It's also interesting that this is typically less expensive than the Seagate IronWolf Pro. It is basically the same drive, so it is CMR and Helium filled. It has a higher MTBF at 2.5M hour MTBF and a five-year warranty, it does show that it is slower than the IronWolf Pro for reasons that are not clear. Given that this is going into a home NAS absolute performance is less important than reliability so either is good with the good warranty. The EXOS has a five-year warranty and a three-year data recovery warranty as well. On Amazon it goes for $528 and $524 at Newegg.
As a note to all, don't use desktop drives unless you like rebuilding. And make sure that you are using RAID10 as with things like Hybrid RAID or SHR, when you do a rebuild, you go through the entire drive and create more errors. Most of these drives have 1E15 error rates so a megabyte is 1E6, a gigabyte is 1E9, and a terabyte is 1E12 so 20TB is 2E13 and that means the chances of en error are 1E15/2E13 so there is a 1% chance that if you read a single 20TB drive, you will get an NBER (Nonrecoverable Bit error). If you have to read say seven drives, it gets worse.