hm, there is -d removable but this seems to deal with it not being present on startup, doesn't mention anything about when it's removed..

removable - the device or its media is removable. This indicates to smartd that it should continue (instead of exiting, which is the default behavior) if the device does not appear to be present when smartd is started. This Directive may be used in conjunction with the other '-d' Directives.

I’m trying to arrange that it is a read only file system, with overlayfs, similar to the ISO.

It does boot, but the duplicate mount is causing problems.

Thanks for the hint; could this explain the oddity I'm seeing?

Essentially, I have a ro store, that I mount at /nix/rostore, and then a writable partition I mount at /. I want to make /nix/store be an overlayfs with /nix/store (on the writable partition) with the rostore as the lower layer. I have something that works, but then the /nix/store directory appears empty if you try to list the files in it. Directly opening the paths is fine.

I can't figure out why I can't get a sane overlayfs mount..

Something like:

fileSystems = {
  "/" = {
    device = "/dev/disk/by-partlabel/nixos-rw";
    fsType = "btrfs";
    options = [ "x-systemd.growfs" ];
    autoResize = true;
    neededForBoot = true;
  };
  "/nix/rostore" = {
    device = "/dev/disk/by-partlabel/nixos";
    fsType = "squashfs";
    neededForBoot = true;
  };
  "/nix/store" = {
    fsType = "overlay";
    device = "overlay";
    overlay = {
      lowerdir = ["/nix/rostore/nix/store"];
      upperdir = "/nix/store";
      workdir = "/nix/ovfs-work";
    };
  };
};