Putting it on secondary IDE means it is free of interferance from your hard disk (which would be on primary IDE). IDE only has a limited bandwidth - and usually runs at the rate of the slowest device on your line - so puting your optical on your primary line would slow down your hard disk to optical speeds (33Mbytes/s). Also, interleaving commands down an IDE line slows down performance due to overhead - so when writing a DVD, the IDE has to read from HDD, then send to writer on the same channel. Both cannot happen at once and the buffer in the writer empties as the HDD is reading. Putting them on seperate channels would allow them to read and write at the same time. Putting it on master is a matter of convenice and usually alleviates any problems with other drives that might interfere if it was set in slave.