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.

I HATE upstairs ... 

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 

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.

I've got a 500 meg drive for the O/S and programs, a terrabyte drive for Audio and a terrabyte drive for samples, local backups and hoarding.

Mod Bod
I've got a 500 meg drive for the O/S and programs, a terrabyte drive for Audio and a terrabyte drive for samples, local backups and hoarding.

Shirly, you must be only having EZD2 on that drive....

Beagle

musicman100
I HATE upstairs ... 

I put my 2cents in upstairs for what that's worth.  I'm not respected upstairs most of the time.  [shrugs]  not that it would bother me at all.  I couldn't care less if they respect me upstairs.
 
Bill respects me.  he even apologized.  that's all that matters. 

We allz respectz ya Beagz......ewez R dawg yo!! (Ca$hDawg too!!)
 
Hey look............a squirrel..........
 
 
 
 
 
(chomp...chomp.....chomp..........yumm becan....chomp......chomp......chomp)
 
 
 

"level of expertise comes to mind..."

Four drives here:-
C: OS and programs - 250Gb
D: Sample Libraries - 500Gb
E: Sonar Projects - 500Gb
L: Seagate External 2TB Backup
 
The exceptions are my Toontrack EZX drum libraries, which leave no install option other than to install on the C: drive. You can move them and alter the registry but I always could see problems down the line there, even if just maintenance.
 
Projects are also backed up using Gobbler cloud server now too, so even if fire / flood or burglary took my DAW and backup drive, my projects are safe.
 
I back up just using Windows back up and restore, to the USB Seagate 2Tb external backup drive.
However, I also have a single folder on my desktop that has subfolders for all my software, libraries and all subsequent updates. This is also backed up once a month and kept in a separate location) I figure that if my DAW totally blew up, this one3 folder would be the enabler to rebuild the entire replacement DAW, as with a 3 drive system, I see back up and restore good only for restoring my current DAW following a repair, or replacement of say, one of the discs.
 
Performance. well, just fine, I cant compare with using just a single drive as I started out with this rig, but even recording with the buffers set really low to get 3ms latency going in, 10ms complete round trip, I have no issues.