Besides, may it be better to backup and restore from recovery mode, using normal tools? E.g. only use fastboot to flash whatever images are required to boot to recovery, then boot into recovery, connect via telnet/nc/whatever and flash the image back using dd. This way one has more control over the process, can skip buggy bootloaders, and may even use a simple .tar.gz containing the rootfs (plus mkfs.btrfs and a script to create the subvolumes, instead of a disk image).