For what it worth...
Here is the way my HDs are:
40gigs:
C:\10gigs\ OS 1 (xp) for, web, etc.
D:\10gigs\ OS 2 (xp) dedicated to the studio.
E:\20gigs\ lets call it a "temporary backup partition" At the end of the day when I know we will continue tomorrow, I simply copy what we recorded to this partition, kind of backup. Not 100% safe, it'S still an internal HD, blablabla, I know.

But its better than nothing!
80gigs:
F:\40gigs\ Dedicated to the studio's audio
G:\40gigs\ Dedicated to MY audio projects. But of course, when the G: is full (it happened a few times, I dont care about using the F: to complete the project. And in fact the next time I'll have the chance to modify my partitions, the 80gigs will be 20/60 instead of 40/40.
oh..and btw, when I get some new software, I always try it on the C:\ part, before installing it in the "studio OS". This way, I know if everything will work OK next rec. session.
And no I dont use Partition Magic. I tried it..uh dunno...2 years ago? was the 5th version I think. And yes, was making partitions, but, sorry, cant remember exately why, it pissed my of the way this software made it. Kind of "simili-partitions".
So I'm still an FDISK dos user... Or makind partitions wile installing Windos is also a good thing.