[mkosacek]

the idea is to spread the I/O's out on 2 or more drives (which lessens the load on each). A hard disk is only capable of so many I/O's per second (IOPs) or so much throughput. SCSI drives can handle random streams of data better than ATA (IDE) or (most) SATA drives due to a feature called "command queuing".

That said, the operating system usually contains a pagefile, which it uses to swap RAM to disk (as virtual RAM) so to have more physical RAM available for running apps (like Pro Tools). To avoid starving either the swap functions, or Pro Tools audio streams, it can help to put them on separate drives. At some point you still reach the limit of a single drive, so the bigger systems have more than 1 drive for teh audio files also.

As for size, it does matter, although most operating systems can exist nicely in 10GB or less (including swap files, etc).

Personally, I use a pair of 250GB SATA drives. One for OS and session backups, the other for current audio projects. so far so good.