Quick paste from my notes:
Audio processing streams a LOT of data. Drives need to give you quick, uninterrupted access. Separate dedicated drives help a lot to let you stream more tracks with less problems. The goal is SMOOTH UNINTERRUPTED THROUGHPUT of data so you wont get clicks, pops or worse, dropouts.
Here's how you want your system set up:
C: (Boot) OS, apps and vsts - your applications and vsts are generally only loaded once and don't hit the disk thereafter HOWEVER your OS will need to do occasional housekeeping work.
(order of secondary drives doesn't matter)
D: Sample libraries
E: thru Z: Music projects and misc data
Partitioning will NOT help you and is, in fact, bad. The arm has to stop what its doing on one partition, lift up and go alllllll the way across the disc to the other side, set down to do its job and then go alllllllll the way back again EVERY time the OS or an app needs to do housework. This mechanical movement is GLACIAL in computer terms and will lead to pops, clicks and dropouts in your audio. AVOID PARTITIONING and go to SEPARATE DRIVES. (the ONLY reason to use partitioning is if you BIOS doesn't support large disks or organizing a disk and then I would still avoid it as partition maps can go bad - or hacked - and then you lose EVERYTHING.)
A 7200rpm drive can stream around 100 tracks simultaneously, so film and orchestra producers will split up music project to multiple project drives every 80-100 tracks.
Because a standard 7200rpm drive can do 100 tracks, expensive 10K and 15K rpm drives are not neccesary. They're generally noiser, too... bad for the studio.
RAIDs are not very efficient on desktop OS's and tests by major magazines (Sound-on-Sound) have found that they only speed up access by 10-15% in the REAL WORLD and are not worth the extra complexity they bring to the desktop. Separate dedicated drives are better.