You can't install GhostBSD on a USB drive without removing the boot USB driver. ZFS will try to boot da1, and your USB is seen as da0 at boot.
Since GhostBSD moves the whole system in the RAM memory, your best course is to dd
the ISO on the USB drive you want to install GhostBSD. Start the installer and install it on the USB stick you booted it with.
The other way is to remove the USB you booted with once you are in the DE GUI and insert the one you want to install GhostBSD on. After that, start the installer.
The boot USB is unnecessary once the GhostBSD ZFS pool is set in the RAM and rerooted.