Hi Beth - a scratch disk is another (physical) hard drive in addition to your boot drive C:.
On Windows machines there's always a lot of disk fragmentation through normal daily use and web surfing. I imagine similar things happen on a Mac, but I don't have that platform so can't speak with any authority. Photoshop and other image editing programs work best when all the data files are in ONE place, and have been subjected to minimal fragmentation.
When you setup Photoshop, you can specify a scratch disk. You are telling PS where to specifically go to retrieve, save and work on your image files. And this physical disk is a different location than the disk holding your program files. PS runs a lot fast and more efficiently when you have a scratch disk. - Stu
Is there a minimal size HD the scratch disk should be? Im in the process of getting some components together for a new computer build, and wasnt sure how big the scratch disk really needed to be. Thanks!!!!!
i started building like 3 years ago.. i paid 300 bucks fora 3.2 P$ >.> on my last computer i paid 90 bucks for a 2.53 dual core OCd to 3.33 
