Mysql can't write to filesystem
On Ubuntu apparmor doesn't let mysql write to directories other than listed in /etc/apparmor.d/usr.sbin.mysqld

apparmor is enabled by default, but on dedicated MySQL servers I don't see much sense in it.

From https://help.ubuntu.com/14.04/serverguide/apparmor.html: 
"AppArmor can be disabled, and the kernel module unloaded by entering the following:

    sudo service apparmor stop
    sudo update-rc.d -f apparmor remove


"