Mount disk to lxd

i try to mount a disk to lxd
i mount in server and format it on ext4 (on ubuntu) den i can mount it as /dev/xvdb1 /mnt/Backup
now i try to mount it (for read and write ) to my lxd
i add:
root:1000:1 to /etc/subgid and /etc/sunuid where 1000 was my user name id in host server
then adduser in lxd
lxc config set “container” raw.idmap “both 1000 1000”
and restart container
Then add mount
lxc config device add “container” Backub disk source=/mnt/Backup path=/Backup
i can see /backup in my lxd but not permission to write. What i missed here?