NixOS Installation Guide/multibootusb

When using multibootusb to create a boot stick, which can boot multiple ISOs and then booting the Nixos ISO from it, there is a problem with mounting the ISO in the first stage.

It says something like

An error occurred in stage 1 of the boot process, which must 
mount the root filesystem on '/mnt-root' and start the stage 2. 
Press one of the following keys:
  i) to launch an interactive shell
  f) to start an interactive shell having pid 1 
     (needed if you want to start stage 2's init manually)
  r) to reboot immediately
  *) to ignore the error and continue

Then press i to launch a shell. In the shell execute

# fdisk -l #To find the block device for your multiboot partition on the usb stick.
# mkdir /mnt
# mount /dev/sdXY /mnt #Use the multiboot partition's block device
# mount -o loop -t iso9660 /mnt/multiboot/nixos-<version>-linux/nixos-<version>-linux.iso /mnt-root/iso/
# exit

Now the boot process should continue as usual.