How does 'Bind' in Docker+Portainer work?

If I add extra storage on (say) /mnt on the host, and use Bind to point /backups in the container to /mnt on the host, does the data get written to both /mnt on the host and /backups in the container (which I don’t want), or does it only get written to /mnt (which I want)?

If the data gets written to both /backups in the container and /mnt on the host, how do I add extra backup storage, please?

(I’m using Portainer for Docker configuration)

Only to /mnt.
/backup is bind mounted to /mnt, so in the container you see /mnt if you go to /backup

Thanks for the reply.
So to confirm - data doesn’t also get written to /backup in the container?

I’m asking because I mounted a USB SSD on the Pi and tried a big backup, but Urbackup kept aborting and retrying. On checking, I found that the data was getting written to the host microcf card instead of the SSD. I also found that the SSD was no longer mounted. I’m now wondering whether the power supply of the Pi (a 3b) is up to powering the SSD.