Mounting external luks drive at boot

Hi,

I just started using dietpi and it looks great, very snappy.

I have an external luks drive with a key which I can open manually no problems. I’ve tried everything I can think of with crypttab, fstab and initfamfs to get it to open/mount on boot with no luck.

# <target name> <source device> <key file> <options>

backup UUID=ec313266-dcd2-45ee-adb1-764a7b3da484 /root/backup.bin luks

# appended to fstab

/dev/mapper/bup /mnt/ec313266-dcd2-45ee-adb1-764a7b3da484 ext4 defaults 0 2

sudo update-initramfs -u -k all

Can anyone point me in the right direction?

Thanks