We originally installed an image from the cPanel website. The /root parition holds the system information and the /data houses the users web and mail data. Unfortunatley the /root partition is NOT installed as an LVM which makes it difficult to expand. What I did notice is that the users MySQL data is actually stored in the /root partition. As this was the majority of the data I decided to move this to the /data partition:
1. Backup the databases
mysqldump --opt --all-databases | gzip > /data/alldatabases.sql.gz
(as our cPanel server is a VPS we are able to take a snapshot)
2. Stop monitoring the MySQL service
WHM -> Service Manager -> Uncheck "Monitor" MySQL Server
3. Stop MySQL
/etc/init.d/mysql stop
4. Move the MySQL data
i) Make the new directory
mkdir /data/var_mysql
ii) move the data from old partition to new partition
mv /var/lib/mysql /data/var_mysql
iii) Change permissions on new directory
chown -R mysql:mysql /data/var_mysql/mysql
5. Symbolically link old directory to new directory
ln -s /data/var_mysql/mysql /var/lib/mysql
6. Start MySQL
/etc/init.d/mysql start