Mounting an extern HD for borgbackup; Part I

storing the nextcloud-aio datas on an external HD the entry in /etc/fstab is
UUID=xyz /mnt/ncdata ext4 defaults 0 1
This works fine.

If I want to save the borgbackup to another HD, I added the following line to the fstab:
UUID=zyx /mnt/backup ext4 defaults 0 0 (last digit also 1 or 2)
Then the Ubuntuserver runs in the maintenance (maybe emergency?) mode.

That’s why I commented out the line for the ncbackup and mount the backup HD manually with
mount /dev/sdb1 /mnt/backup
The server starts well and the command df -Th shows, that the HD is mounted well.
/dev/sdb1 ext4 3,6T 148G 3,3T 5% /mnt/backup

My question is: Why the entry from
UUID=zyx /mnt/backup ext4 defaults 0 0
leads when starting the server into maintenance mode?