'--asroot' option has recently been removed. https://projects.archlinux.org/pacman.git/commit/?id=61ba5c961e4a3536c4bbf41edb348987a9993fdb
Need to check if arch-install-media runs as root, know of course you could add user but it might make things a little difficult if you need an aur application (such as broadcom-wl). MrG
