I’m having good results with a NanoPi Neo2, with the “NAS Kit” Accessory board, allowing me to attach a 2.5" SATA SSD drive.
While this board might not be the absolute cheapest ARM board out there (I say it’s still darn cheap), the Armbian builds for it are “Supported” by Armbian, and fully stable-feeling. (BTW: I recommend spending the extra $10 and get the 1GB of RAM, not the default of 512MB RAM.)
If you are a conservative person, wanting a very stable board, with a single very stable SATA-attached SSD, then I recommend this arrangement. Almost all other SBC’s out there with some SATA connectors do not have the status of being officially “Supported” by Armbian (at least, not yet). Sure, the Helios 4 is fully “Supported” by Armbian, but is much more expensive than the NanoPi Neo2.
After some substantial testing, I was able to get sustained sync speeds of ~15-25MB/sec with syncthing, and this is almost exactly same speed as what you’d get if you ran a Samba server on this NanoPi.
Note: I had disk I/O troubles attaching larger-capacity 3.5" SATA drives in USB enclosures, mind you. See here for more on that. I only recommend one 2.5" SATA drive attached to the NAS-kit board. If you attach disks to the USB ports, you were warned. (Hint: watch /var/log/syslog for Disk I/O error messages). Having said this, perhaps it was the poor quality of my disk enclosures themselves which were to blame. I wasn’t able to nail down what was to blame. But I do totally trust the one SATA connector on the NAS kit board.
Call me paranoid, but I’ve lost all trust in USB-attaching any storage for use in a file server. I’ll only use at least SATA-attached storage now (or better, like M.2), and it had better be known to be a very stably-performing SATA, in the way of kernel support (as in, Armbian calls it “Supported”, and “Stable”).