MySQL switch between data directories for individual databases
Rather than retype a lot information, here are some posts I made in the DBA StackExchange before on how to move MyISAM tables to different folder and disks


Dynamically changeable database
Can I specify a different disk for a specific MySQL table?
Mail alert from MySQL when databases are edited (I suggested creating a MyISAM-based general log that would stored on another disk. I did this for a client long ago and collected over 125GB of log info)