The tutorial will teach you how to relocate /nix to a new device on non NixOS Hosts. A summary of how this could be done on NixOS is at the bottom. This has not been tested on NixOS but there may still be a way:

On *BSD/Linux Host

The device that /nix is on has no more space

$ du -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       182G  180G    2G  99% /

Stop nix daemon

$ sudo systemctl stop nix-daemon.service

Move backup /nix; mv /nix to /nix.bak

$ ls -alF /nix
total 1068
drwxr-xr-x    4 root root      4096 Jan 22 02:43 ./
drwxr-xr-x   26 root root      4096 Feb  8 12:17 ../
drwxrwxr-t 1555 root nixbld 1077248 Feb  8 12:05 store/
drwxr-xr-x    4 root root      4096 Jan 22 02:43 var/

$ cd /
$ tar -afp /nix.tgz /nix /nix.bak
$ sudo mv /nix /nix.bak # in case something goes horribly wrong

Mount new device. You will need to add this device to `/etc/fstab`

$ mount /nix
$ du -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb1       300G    0G  300G   0% /nix

Restore /nix from /nix.tgz; verify contents

$ cd /
$ sudo tar -xf /nix.tgz 
$ nix-store --verify --check-contents

Start nix-daemon

$ sudo systemctl start nix-daemon.service

NixOS

Template:See also Add rescue-boot.nix to your imports section.

nixos-rebuild (and pray you have ~300mb free on /boot/)

run the installer from grub at any time.

adjust your configuration.nix so it expects /nix to be at the new place, and nixos-rebuild boot, so it doesnt break after you move it

Now reboot and move it.