I think you need at least 3:
OS
Audio
VSTi
better with 5:
Data (mostly install stuff on the DAW)
Backup (I call this one "Studio", finished mixes, images)
I like to put the waveforms on the 5th drive.
And of course an external (externals) for project backups (and redundant images).
Tom

I have two one for OS and programs including VSTi's etc. One for audio data. I do not use any large sample libraries that stream from a hard drive, so no opinion there. Most laptops do not give you the option of more than one drive, so not a lot of choice for those.
 
One argument against putting data and system on the same drive is that many imaging programs do not let you choose to exclude the data from the partition image, making a gigantic backup drive necessary if you do a lot of image backups. You can use partitions on the single drive to solve that problem, but there may be some additional degradation of performance from partitioning if the drive heads need to jump partition boundaries between system and data frequently.
 
I have to say that I have not really had a performance hit that I can attribute to drive access problems, but I do not do enormous projects, and I have an excessive amount of memory installed on my DAW.

bayoubill
Kool! A poll!
 
OK, I have an C Drive for Programs, an D drive for recovery, A  E drive for DVDs/CDs, and an S drive for storage. 
 
(1+1+1+1= 4)
 
I Am not sure but I would say I have 4 drives for stuff 
 
I will read this Fred carefully
 
P.S.  Sonar is running noticeably better with MM100's set up!!! THANKS AGAIN!

SEAFOOD PLATTER???????????????

1,234 - One for each project.
 
Sometimes I forget to label them and can't find the one I wanna work on... so I just gets another one.
 

 
I can tell you don believe. OK I only have 2. That way if the OS drive crashes or needs to be re formatted all my projects are safe.

soens
OK I only have 2. That way if the OS drive crashes or needs to be re formatted all my projects are safe.

And if the other drive fails, then you can still log on to post about it. 

I HATE upstairs ... 

SEAFOOD PLATTER???????????????
 
YES!! 

Cooter Brown Seafood is open 11am-1:30pm Tues.  Wed. - Thru. 11am - 7:45pm and Fri. - Sat. 11am - 8:45pm
I Am Shirley goin!!!
but not shure when 

3
1 = OS
2 = Audio
3 = other art work

I saw the thread title and thought for a moment that they'd finally enabled the polling feature...sadly, no. Polls aren't nearly as much fun if you have to read multiple pages to get the data.
 
Oh well. Four drives in use here: O/S & apps, audio, samples and backups.