Seems like I'm hitting the same issue with hdd image. iso-hybrid is working fine.
config (not hook or any other modification after this) : lb config --distribution stretch --binary-images hdd --architectures amd64 --linux-flavours amd64 \ --archive-areas "main contrib non-free" --apt-indices false --security true --updates true \ --mirror-bootstrap http://ftp.fr.debian.org/debian/ --mirror-binary http://deb.debian.org/debian/ --bootloaders syslinux \ --bootappend-live "boot=live components timezone=Europe/Paris locales=en_US.UTF-8 keyboard-layouts=fr hostname=rescue \ username=user noprompt noeject autologin" Result : 0+0 records in 0+0 records out 0 bytes (0 B) copied, 7,7539e-05 s, 0,0 kB/s !!! The following error/warning messages can be ignored !!! P: Mounting /dev/loop2 with offset 0 1+0 records in 1+0 records out 440 bytes (440 B) copied, 0,000428679 s, 1,0 MB/s P: Mounting /dev/loop2 with offset 1048576 mkfs.fat 4.1 (2017-01-24) mkfs.vfat: Partitions or virtual mappings on device '/dev/loop2', not making filesystem (use -I to override) P: Begin unmounting filesystems... P: Saving caches... Reading package lists... Done Building dependency tree Reading state information... Done