You're better off making sure you clean up the logs folder, freeing up space, run a full fsck and tune2fs series on the partition, then copying/expanding.
You really *ought* to be able to just take the new drive, extract the rootfs, insert mac_addr, boot, pacman -Sy udev-oxnas, *reboot*, mount nand, copy needed binaries (which NO WE CANNOT DISTRIBUTE), *reboot* and pacman -Scc, pacman -Syy, pacman -Syu.