Hmmm, not exactly a programming question but wth. You are doing it right. All that needs to be done is datadir parameter changed and all the data files moved to the new location. Though, you must keep in mind these few things :
- new directory exists at the moment you start db
- your mysql database was copied to the new directory (just copy the files)
- your server really uses the file that you are modifying
In the case you have already deleted your old mysql db - which basically means that you lost all your users and settings - you need to recreate it. Use mysql_install_db script to do it. Once done, make sure you have proper ownership on these drectories (mysql.mysql). If none of above works - call ghostbusters