If you have ever seen the message "Could not complete your request because the primary scratch disk is full", you'll know how frustrating Photoshop can be when it gets low on operating space.
First of all, what's a "scratch disk"? Photoshop can work with some very large files - high resolution scans can head towards 25Mb-100Mb each. It therefore needs to be able to handle the memory requirements of these files if there are several open at once, or when you want to be able to undo a whole series of steps. To cope with the likely situation that there will not be enough available RAM to hold the images, Photoshop goes direct to the hard disk to use it as overflow space, bypassing the operating system. Although both the Mac OS and Windows also use so-called "virtual memory" in ordinary operation, Photoshop likes to do its own thing, mainly for performance reasons. This is the "scratch disk" - overflow space for Photoshop, and you can allocate up to four disk drives for the purpose. However, there are a few rules to observe.
Scratch disks must be fixed physical hard drives or partitions - no network drives or removable drives - Jaz, Zips or CD-RW - can be used.
You should allocate your largest, emptiest, fastest hard drive as the "primary" scratch disk - the others won't get used until this one fills up.
Keep your primary scratch disk fairly empty if you can, and certainly keep it well defragmented and free from errors - use a disk checking program such as Windows' ScanDisk.
If the primary scratch disk fills up, you will have problems - hence the dreaded error message: "Could not complete your request because the primary scratch disk is full". This is often a Catch-22 - you will not be able to close the file because there is not enough temporary space, and unless you can delete something else from the drive you can't find enough space to save the file. You need to make sure that your primary scratch disk has free space equivalent to around five times your largest file and preferably equal to around twice the amount of RAM fitted in your computer.
Why twice the RAM? A little-known fact is that when Photoshop starts up it checks the available scratch disk space. If it can only find (say) 36Mb of scratch space, it will only use 36Mb of RAM, regardless of what you may have set in your preferences (see above). So if you are low on primary scratch disk space not only will you have trouble because of a lack of space for temporary files, but Photoshop will be using less real RAM to start with, making it more likely that it will need to use the scracth disk! So keep your scratch dirve empty and performance will be maintained.
If you can, buy an extra hard disk and use it exclusively for Photoshop. It sounds like overkill, but big hard drives are cheap (under £100 UK or $140 for 20Gb), and both Photoshop and your other programs will run much better.