Problem after moving mysql database

Hello,

When the IT dept set up my Linux server with MySQL they set the data directory to /var/lib/mysql

Now the system disk is full.

So there is a disk with loads of space which i have copied of the database to using:

cp /var/lib/mysql /data

then i edited the my.cnf file and changed the row that Points to the database to:

datadir = /data

I deleted some old log files to create some space.

Then i restarted the server.

But i now get an error message in mysqld.log that says:

Fatal error: Can’t open and lock privilege tables: Table ‘mysql.host’ doesn’t exist

How can i solve this?  I have seen examples that say you can use:

mysql_install_db –user=mysql –ldata=/newlocation

but i just want to be certain i am not doing a total new install by type in that command.

I know nothing about Linux/MySQL so keep you answers to beginner level!