[ogfx@ogfx:~/ogfx-nixos-rpi4]$ sudo nix-collect-garbage -d
[sudo] password for ogfx:
removing old generations of profile /nix/var/nix/profiles/system
removing old generations of profile /nix/var/nix/profiles/per-user/ogfx/profile
removing old generations of profile /nix/var/nix/profiles/per-user/root/channels
finding garbage collector roots...
removing stale link from '/nix/var/nix/gcroots/auto/0ar82dh1c53jyr3hm71jpi7m6xx78hp2' to '/tmp/nix-build-4949-0/result'
removing stale link from '/nix/var/nix/gcroots/auto/z84cll6p6b6dcwwshzfqj86rmnl784q0' to '/tmp/nix-build-4918-0/result'
removing stale link from '/nix/var/nix/gcroots/auto/gssl9zrl52540j755j8s36vv8cgdbdmg' to '/tmp/nixos-rebuild.kRZ0Iy/nix'
removing stale link from '/nix/var/nix/gcroots/auto/yvpmd5pxgkb46dd2vkri8ihsnd27z88w' to '/tmp/nixos-rebuild.kRZ0Iy/nix.drv'
deleting garbage...
deleting '/nix/store/trash'
deleting unused links...
note: currently hard linking saves -0.00 MiB
0 store paths deleted, 0.00 MiB freed

[ogfx@ogfx:~/ogfx-nixos-rpi4]$ sudo nixos-rebuild switch
building Nix...
building the system configuration...
updating the boot generations directory...
cp: error writing '/boot/kernel.img.tmp': No space left on device
warning: error(s) occurred while switching to the new configuration

[ogfx@ogfx:~/ogfx-nixos-rpi4]$ du -h /boot/
73M     /boot/old
128M    /boot/