Hello,
It seems you have enabled quota support for /backup drive. You can delete those files. quota is not advisable in backup drive. You may also need to remove it from /etc/fstab if it is there.
If your fstab contains an entry something like this
/dev/sdb /backup ext2 defaults,usrquota 1 2
please make it as
/dev/sdb /backup ext2 defaults 1 2