How about 2.5 inch drives? I'm thinking about: https://cdn-reichelt.de/bilder/web/xxl_ws/A300/RPI_NAS_4XSAT...
However, I did look for 2.5" drives (the server comes with a 2.5" backplane) and there don't seem to be many "consumer" drives in this format that are both high capacity (>2 TB) and non SMR (I'll be running ZFS). I'm also not looking to spend very much, so SSDs are out.
[0] Seagate specs for 2.5" Barracuda: https://www.seagate.com/www-content/datasheets/pdfs/barracud...
The startup current is 1.2 A under 5V. So if you're running four of them, you'll need 24 W just for the drives. Not sure if the "cheap" enclosures are able to stagger drive startup.
My Gigabyte motherboard cannot, so it would require a PSU large enough to drive both the board (CPU + RAM + etc) and the drives for a little while. So with four drives, you'd be looking at 50 W to be sure it fits.
SSD and low cost is still possible, but one has to make compromises in terms of storage size. A 1 TB network drive would be enough for my use case, be it 4 x 256 SSD's or one big.
So in the end the rated W can be quite misleading, particularly for home use where the NAS will tend to sit idle a large part of the day.
1Gbps is the max. anyways for my home setup, no matter how fast the drive is and I have a backup for important stuff, I'm not dependent on enterprise NAS drives.
I want: extremely small, low cost and very low watt usage.
If your use case / setup allow you to deal with the complete loss of your array (which, granted, is not 100% likely) or don't use ZFS, then I suppose you could look at Seagate's 2.5 Barracuda line. They're relatively cheap given their capacity and I don't think they're particularly unreliable in and of themselves.
[0] There are many people talking on the internet about ZFS performance with SMR disks. Here's a quick find:
https://arstechnica.com/gadgets/2020/06/western-digitals-smr...
It's not super much space, but any two drives can fail, it's rather cheap and low powered + passive cooling should be possible.