Sure Lars - you going to pay the recovery bill when his main drive directory flames out or a bad block develops on the outer part of the drive
If you are using Photoshop assigning a small empty Scratch partition to the first part of the drive (2 gigs is fine ) keeps PS happy.
After that a decent OS partition next and then a Data partition. Make sure you give X at least 30% breathing room so perhaps 20 gigs for a system partition and then the rest for data unless you plan a lot of downloading in which case a larger boot partition might be advised.
OS9 can go anywhere - it's not a bad idea to have it elsewhere as sometimes it's a lifesaver.
Are either drive an 8 meg cache??