[///AznTi]

If it was my drive I would just leave the OS drive alone and not partition it.

This way all data written is one large cluster and not so separated.
But thats how I do things. Its really up to you.

Question though, do you really need to create another partition on your main drive or are you just trying to justify why you are wasting the rest of the drive for the OS?

You should do what you originally want. Use the other two drives for back up and store all files there.

For me, I do not like to go over 80 gigs on a main drive.
I'll straight up install it and leave the rest of the space for programs and a separate folder for my VST directory.

Then all my external drives which are 200 gigs are partitioned 4 times NTFS.

I simply store all data on there.

Hope this helps.