Is an external root partition supported (hybrid boot mode)?

Older Raspberries that do not yet support USB boot mode, can be configured in such a way that only the boot partition is stored on the SD card and the root file system on an external USB disk.

raspiBackup supports external root partitions and an external root partition is also backed up.

In normal backup mode, the two Raspberry Pi OS partitions /boot and /root of the system device are normally backed up. However, if the root partition was moved to an external partition (USB stick, USB disk, ...), this external partition is backed up as well as the boot partition from the SD card. If this backup is restored, the option -R must be used to specify the target device for the external root partition required in hybrid boot mode.