I have a 173G boot/Windows/programs disk and a 3-disk RAID5 array for data. I know the scratch disk should be on a 5th disk, but I don't have one. If I put the scratch disk on the boot drive, as far as performance goes, does it matter if it's in a separate partition or not?
#1
Given a choice the boot drive is your worst choice if your paging file is there (which it is by default) because PS and OS may be fighting for access simultaneously. Partitioning is irrelevant because there's only one head.
Any other drive is better it doesn't matter if data is residing there. A dedicated drive is preferable but not essential, and the faster the better.
#2