[Warning] Can't create test file /mnt/temp/some_dir/blah.lower-test
...
[ERROR] InnoDB: The innodb_system data file 'ibdata1' must be writable
...

/data/ r,
/data/** rwk,

sudo service apparmor reload

sudo chown -R mysql /var/lib/mysql
sudo chgrp -R mysql /var/lib/mysql