I was thinking it would be nice to have for backups, but maybe even going as far as mounting ~/home to be able to run from multiple machines. I’m really asking where to get started looking without crashing into marketing department nonsense and search engine steering bias.
With Merlin (another open-wrt variant specifically for Asus routers) you can simply connect a hard drive via USB and then enable it as either FTP or SMB storage (even simultaneously).
In windows you can mount the ftp volume as a local drive, from mobile I use X-plore file manager (also via ftp), and on my Nvidia Shield I can mount it via smb and watch movies directly. 4k 10bit, no problem.
Mobile access screenshot: https://imgur.com/a/0ElHYbU
I’m using the Asus RT-AX88U with whatever Merlin firmware is current, but it’s been working ever since, and straight out of the box. Other than activating the function and setting passwords, I really didn’t do anything.
I second Merlin! I did this for a few years before I got an actual NAS set up. It’s handy for simple network shares. The only caveat, is depending where you have the router located, heat can cause trouble. And also, since my router is an older model with less memory (AC-68U), sometimes it would freeze up if left running too many days with no reboot. That shouldn’t be an issue now, as they all have more memory. Just don’t stick your router inside a bookshelf where heat can gather, and you should be good to go. It will likely be lower speed than a lot of other NAS options, though, so it truly depends on your needs. But honestly, Asus-WRT-Merlin for the win. I used the SMB option, because you can browse right to it with no special software required from Windows, OSX, or Linux. There are even Android file explorers that can connect right to an SMB share (Solid Explorer rocks).
I ran Merlin for a couple years on an RT-N66U. Eventually switched to Tomato and was much happier with it.
It died a couple years ago. Replaced it with a Unifi Dream Machine. No ragrets.