You can’t backup an Aurender to an external HDD connected to the Aurender. You can copy from an external HDD on the Aurender to the internal storage.
If you want to backup the X100 files you will need to copy from the Aurender to a computer shared drive, NAS, or an external HDD on your computer that you have shared.
If you want to backup the X100 files you will need to copy from the Aurender to a computer shared drive, NAS, or an external HDD on your computer that you have shared.