What Cleophus said, with a couple of extra points:
While 3.5” drives are the right choice for sheer capacity, you’re a little off regarding the capacities of 2.5” drives—they get considerably bigger than 1TB. Western Digital’s 2.5” drives max out at 2TB, I believe, and you can get a 5TB 2.5”’drive from Seagate, though it’s 15mm tall rather than the laprtop-standard 9.5mm. But the 15mm drives fit in 3.5” drive sleds just fine, albeit most sleds require a cheap adapter.
2.5” SSDs are commonly available in up to 4TB sizes. But it won’t make sense to buy 2.5” SSDs much longer, and they’re restricted by the SATA interface to a nominal 6Gb/s. But watch out—so are some M2 drives.
What you want for maximum speed are NVME M2 drives. They’ll transfer about 2000 MB/s—that’s 16Gb/s, or about four times the real-world maximum of A single SATA SSD, and more than ten times the maximum transfer rate of a single spinning SATA hard drive.
MSATA M2 drives are fairly common, but they’re as slow as regular SATA SSDs. Many M2 slots will accept both MSATA and NVME M2 drives, so watch out.
(All of this is for consumer-level hardware. You can buy bigger, faster 2.5” enterprise drives, but these use the SAS interface, not SATA. Enterprise-level SSDs also have higher capacities and a unique interface).
The first question you need to answer is: “What do I want my NAS box to do?”
It used to be that four spinning disks in RAID 5 have you redundancy (in case of drive failure) and sequential read speeds that, over gigabit Ethernet (at about 110 MB/s in the real world) were similar to the speeds you’d see on a local drive.
Now, that’s painfully slow. I use my own NAS as a file server for things I don’t need in a hurry. I also use it for incremental backups for several different machines. It used to be that you wanted drive redundancy because spinning drive failures are so common. RAID1 or RAID5 ensure that you can get to your data even if a drive in the array dies.
But SSDs are reliable enough that availability isn’t a big concern for me. My NAS is in flux right now, but eventually it will have the following storage devices:
- One small NVME drive for the OS
- One 2TB NVME SSD for serving files
- One 4 TB 2.5” drive for incremental backups of local files, including those on the NVME SSD, as well as incremental backups of machines on my network, and
- One 8 TB HDD for incremental backups of the 4 TB drive
There used to be a fair amount of geek prestige in having, say, 8-12 TB of RAID5 (or whatever) storage at home, but big, fast hard drives consume 6-10 watts each much of the time. Six drives sucking down eight watts each cost me about $72/year to keep in operation. In my current setup, I only spin up my HDDs when it’s time to do a backup, and they’re spun down shortly thereafter. That saves a lot of power compared to a spun-up HDD array, and one NVME drive is substantially faster than a six-drive RAID0 array. Of course, I’ll need 10Gb Ethernet to take advantage of those speeds, but that hardware is getting cheaper.