Why does it say my "scratch disks" are full?

The OS needs an absolute minimum of at least 10 - 15% of your entire hard drive space to be empty at all times in order to function properly. So, how large is your hard drive and how much space do you have left? Please post that information.

The free space on your hard drive can be used as working memory (supplementing RAM) if there isn't enough RAM to do what you're trying to do. Mac OS X does this by growing its virtual memory swap files. Photoshop has its own virtual memory which grows on top of what Mac OS X does. If you don't have enough RAM for the kind of edits you're doing, and especially if it's a large image with layers, your disk space can go quickly.

OK so the figures you added, is that before or after you open Photoshop? And also how much RAM do you have installed?

Also it can build up over time, as in the two hours you were working. One thing you can do is keep a Finder window open in the background and check the free space at the bottom as you work, to see how fast 27GB turns into 0GB free.

Okay, so that is not the problem - how much RAM do you have?

Open Activity Monitor (Applications > Utilities > Activity Monitor) and check the processes as well as how RAM is being used (at the bottom of the window).

You mean you only have 1.75 GB RAM? That is an odd number - can you make sure please and post a screenshot like mine below. Go to the Apple, click on about this Mac and then on more info. In the resulting window scroll down to memory and post a screenshot or post the total amount of RAM.

FWIW, my info at the bottom shows this (and I have a total of 4 GB RAM):

Okay, so it really is 1.75 GB - what year/model is your machine? And does it have one or two memory slots?

You will need to increase RAM - that is if your machine will support that (which is why we need to know the model/year). Photoshop is a RAM hog and you are pushing it with those numbers...

I think there are 2GB in that Mac. I don't think Apple shipped 2009 MacBook Pros with less than 2GB, and the numbers (1.52GB Used + 493MB Free) add up to about 2GB.

But the conclusion is the same. 2GB is bare minimum for high-end graphics usage. I wouldn't want just 2GB if I was using Photoshop, Aperture, Final Cut, etc. Also, if you have less than 4GB, you aren't really getting the most out of your 64-bit CPU cores.

Photoshop wouldn't need much RAM if you were working on web-sized images, like 800x600px. But if you are opening 10+megapixel images and add layers, 2GB is just not going to do it.

RAM for MacBook Pros is cheap (see macsales.com), so if you're going to edit big images often, get that Mac up to 6GB or 8GB RAM.

I think this is the right page for your model. If it is, maxing out at 8GB would only cost you US$50.

http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3

One other way to avoid filling up your disk: You can connect a fast external hard drive (FireWire 800 preferred) and set it as your Scratch Disk in the Photoshop preferences. If you do this, Photoshop will fill up the external disk with its temporary scratch files, instead of your internal disk. It will also be faster because of the reduced burden on the internal hard drive. Photoshop pros usually speed up the app with both approaches: loading up their Macs with tons of RAM, and using outboard scratch disks. Then they can handle big files fast.

Since you say you spend a lot of time in Photoshop, you might want to read this too.

Optimize performance | Photoshop CS4, CS5