I have cleared one of my 500Gb internal drives in order to allocate it exclusively to CS3.
In CS3/Edit/Preferences/Plug-Ins, I have ticked the box Additional Plug-Ins Folder and browsed to this clean drive.
Question: In CS3/Edit/Preferences/Performance the Scratch Disks box list this drive along with my C drive and one other.
Since I have allocated this drive under Plug-Ins, should I now untick the box next to it in Performance?
The question is not really clear, but if you re asking if you should assign the hard drive containing plug ins as a Scratch disk, then I would say No. You should better assign a hard drive with no use, other than being the Photoshop scratch disk. If the size of that disk is 500gb as you say, then I d say you need no second scratch disk.