Backup your database
Before making any changes that may cause problems (such as upgrades), you should always back up your MythTV database.
The mysqldump command
$ mysqldump -u <myth_user> -p <myth_db_name> > mythdatabase.bak Password: <myth_password>
Be sure to replace <myth_user>, <myth_db_name>, and <myth_password> with the appropriate values.
The resulting file is a plain text file that can be fed to the mysql command in order to restore a database. This file can, and should, be copied to another machine in case something really bad happens and the entire disk needs rebuilt.