Are you trying to do this on a GL-Inet device?

Yes, this is an AR-750S.

Shouldn’t need sudo, as OpenWRT is running as root/admin, and the default shell is not zsh, it’s ash provided by busybox

I created a new user and installed both sudo and zsh, and made zsh my shell. I also tried doing this logged in as root and got the same result. No user can echo to the procfs file while the nfsd is running, and if you stop it, all the procfs files evaporate.

I’m just trying to use the 128GB SD card I have in the router’s slot as a sort of temporary sharing datastore for a Mac and a Linux machine.