How to fix the error "unknown variable datadir" in mysql on raspbian?

I want to safe the Databases on an Usbstick and not on my RaspberryPi.

First I did:

• service mysql stop

• mkdir /media/pi/EMTEC/mysql

• nano /etc/mysql/my.cnf and changed datadir=/media/pi/EMTEC/mysql

• cp -Rv /var/lib/mysql/* /media/usb4/mysql/

• chown -R mysql:mysql /media/usb4/mysql/

which worked without problems

My my.cnf:

[client-server]

!includedir /etc/mysql/conf.d/

!includedir /etc/mysql/mariadb.conf.d/

datadir=/media/pi/EMTEC/mysql

When trying to restart the mysql service I receive:

mysql: unknown variable 'datadir=/media/pi/EMTEC/mysql'

How do I fix that error?