If I remember correctly you said you have one gig of memory? That isn't enough to do much other than web browsing, email, text files, etc. I think the only machine that you can have one gig of memory with is the mini which also shares memory with it's GPU. You can ad memory to it, but to do so might void your warranty. You will have to go online to find instructions on how to do it (involves putty knives, not for the squeamish).
That means after your system takes it's share of memory, there is not much left for Photoshop so just about everything you do is done with the scratch disk. If you meant 1 gig of disk storage, that also is not enough.
If you have one hard drive, than that hard drive is your scratch disk. That means that whatever is left over on your hard drive that is not used to store the system itself, application files, and work files is what is available for your scratch disk. If you have items in your trash that you haven't emptied, that also is disk space not available for your scratch disk. The more you have on the drive designated as your scratch disk, the less you have available of it to be used as virtual memory (scratch disk).
As I understand it, the Mac OS has a built in optimizer that basically defrags your hard drive automatically. The default is to do this at around 2:00 am. If you shut your computer down on a regular basis for the night, it doesn't get defragged.
As mentioned by another poster, PShop writes a temporary file to the hard drive or partition you've designated as the scratch disk. This file is deleted automatically upon quitting PShop (unless you happen to crash while in PShop, in which case you might see one or more files called 'recovered file' in the trash—empty your trash).
So, since the scratch disk in PShop is simply a hard disk or disk partition you designate as the scratch disk, the way to clean the scratch disk is to remove any unecessary files from the hard drive or partition you've chosen to be the scratch disk—(as in back up files elsewhere, then throw them in the trash and empty your trash). If you only have one Hard Drive and it is getting full, your PShop scratch disk is getting smaller.
Your options are any or all of the following:
1. Ad more RAM if you can. It's relatively cheap now (don't buy Apple memory—it's way overpriced).
2. If you only have one Hard Drive for everything, including PShop scratch disk, add another Hard Drive and keep it empty for use solely as the scratch disk or buy one big enough that a decent chunk of it can be partitioned and used as the scratch disk but not for storage. If you have a mini you obviously need an external hard drive as your dedicated scratch disk. Drives can also be had for not much money (try newegg.com)
3. If you can't/won't get a dedicated drive that you keep empty for use as the scratch disk, back up and remove files from the disk or partition you are using as a Scratch disk regularly.
4. Close down all other applications you are not using, especially Safari, it drains system resources even in the background (at least it use to unless Apple has corrected this)
5. Be aware of how you work. While working in PShop, when you get to a point in your work that you know you won't need to go to previous stages with "History" save your file and go under, edit/purge. You can purge (free from memory) Undo, Clipboard, History, or All. If you copied something to your clipboard, after you use it, "Purge" the clipboard.
- The more memory and storage space the better. Cost is relative to time. Struggling with too little RAM and too little storage is expensive in and of itself. The mini is due for an update, probably in Jan or Feb. If you're on a very tight budget and what you have is a year old mini, you've got a decent amount of use out of it. Consider upgrading. If they improve the graphics card and allow more than 2 gigs of RAM in the updated mini, it might be worth buying a new one for you if you plan on using it for PShop.