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How to Clear Your Scratch Disk


Scratch disk full - How to clear your scratch disk

If you are a Mac OS X user who regularly works in programs like Adobe Premiere Pro, Final Cut Pro and Adobe Photoshop, you’ve probably encountered errors relating to a “scratch disk” — the most common being Photoshop’s refusal to open along with the message ”scratch disk full”.

If you’ve stumbled on this page because you’ve experienced this issue, you’re in luck: we’re going to show you how to easily resolve your scratch disk problem and, as a bonus, keep your Mac clean and performing at its best.

This guide is going to be focused on the specific issue in Photoshop, as it is the most common, but these tips will be beneficial for most programs that make use of a scratch drive.

In a few minutes your annoyance and frustration will be gone and you’ll once again be using Photoshop to your heart’s content.

What is a scratch disk?

Before we highlight some solutions, it would be a good idea to have at least a general sense of what a scratch disk actually is. When you are using programs like Photoshop, they require a working space called virtual memory or cache memory, where temporary project files are stored and accessed.

When Photoshop can’t complete a task using your RAM, it uses hard drive space as temporary virtual memory instead. This hard drive space is called a scratch disk. Photoshop allows you to assign several scratch disks: that way, it has more space at its disposal.

As you work, particularly with large images and lots of layers, your space can be eaten up by the application’s temp files in no time at all.

How to choose scratch disk location within Photoshop

It’s important to note that when you set up Photoshop, you will have the option to assign the scratch disks to the drive of your choice. We highly recommend not using your system disk drive, unless you have no other alternatives.

"Your scratch disks are full" - What does this mean?

The dreaded “scratch disk full” message is probably the reason why you’ve stumbled upon this guide. Mac users receive it when Photoshop (or any of the other programs) has used up all the space on the hard drive that has been assigned as the scratch disk. This used up space is temporary and separate from your actual project file. The problem is that Photoshop doesn’t always get rid of these temporary files when they aren’t needed anymore, which is more like a permanent problem if the user can’t find a solution. So how would you go about emptying your Mac OS X scratch disk? Let’s find out.

#1 - Change your scratch disk

It can be a hassle but if you keep an eye on the available space on your scratch disk, you can do something when the space is becoming limited. It’s also worth noting that problems don’t just occur when the drive is completely full, hard drives should ideally never be above 85% capacity. Once you pass that threshold you’re asking for problems. So, you’ve identified rapidly depleting free space on your Mac hard drive — now what? Well, it may be a good idea to change your scratch disk to another drive in your system (just not the system drive, as that is one drive you really don’t want to slow down). If you don’t have another drive, you could purchase one, with SSD being the best option for Mac OS X scratch disk purposes.

Scratch disk purposes

Follow these steps to change your scratch disk in Photoshop.

  • Click on the Photoshop menu
  • Go to Preferences and then Performance
  • Tick the checkbox to select or remove a drive as the scratch disk
  • Click OK
  • Restart Photoshop

#2 – Clear your disk space

If you don’t have another drive or don’t want to buy one, then it’s time to do some spring cleaning. Look at the drive contents to see if you can delete anything you don’t need. Is there hard disk space that can be cleared? Usually there is, so go ahead and delete your old files and free up some space.

Alternatively, you can simply transfer files to external drives, DVDs or cloud storage solutions like Dropbox.

Take a look at our How to Clean Your Startup Disk (10 Ways). Inside we have lots of practical advice for Mac OS X users who want a clean drive that performs well. Even if your scratch disk isn’t your startup drive, and we hope that is the case, there are many tips to help you spring clean like a pro.

#3 – Clear your Photoshop cache

Photoshop has an in-built solution for deleting specific Photoshop caches. These pesky caches, when they aren’t removed automatically by the program, can grow and take up a lot of space on your scratch disk.

To use this tool simply enter Photoshop in Mac OS X and with an image open:

  • Click the edit menu button.
  • Hover your mouse over “purge” to reveal 4 different options.
  • Undo, Clipboard, Histories, All. If an item is greyed out, that means it has already been deleted.
  • Select the specific item you want to delete or select “all” to delete all of your caches.
  • You will be warned that the purge cannot be undone, so make sure you don’t need a previous version of one of your projects and then click OK.

A good way to keep Photoshop cache from piling up is running regular disk cleanups. That’s pretty easy to do with an app like CleanMyMac 3. It lets you clean up your whole Mac, but the best things is, it can remind you to do that before you get the “scratch disks are full” error again.

Here’s how you can schedule regular cleanings with CleanMyMac 3:

  • Launch CleanMyMac 3.
  • Click CleanMyMac 3 icon in the menu bar and go to Preferences.
    CleanMyMac 3 menu bar
  • Select the Scheduler tab.

CleanMyMac 3 Scheduler

In Scheduler, you can choose how often you want to be reminded — say, every week. When a cleaning is due, you get a pop-up reminder, launch CleanMyMac, and just let it do its thing. That way, you’ll never forget to clear Photoshop cache and tons of other system junk on your disk.

You can download CleanMyMac 3 for free and see how it works (it’s actually perfect for those thorough disk cleanups we mentioned in the previous step).

#4 – Delete your temp files

When it comes to wasting space on your scratch disk, the worst culprit is often Photoshop’s own temp files. If Photoshop isn’t going to get rid of them, I guess you’re going to have to do the job yourself.

To find them you’ll need to look for files that begin with “pst” and then a string of numbers followed by the file extension “.tmp”. Do a search for “Photoshop Temp” with a space between the two words.

You can just search your scratch drive, but to make sure everything is found, rather perform the search on the entire computer. It may take a couple of minutes for the search to complete, so pop into the kitchen for a quick snack while you wait.

Photoshop temp search

When you return, you should have a long list of files. If your work is saved and the program is closed, you can safely delete these files and watch as your scratch disk space is reclaimed.


Now that you know what a scratch disk is and what happens when it gets full, you can ensure that you never receive the dreaded “scratch disk full” error again. By following the easy tips in this guide, you will not only keep Photoshop running smoothly, you’ll have a better performing Mac across all uses.

Whether you solve the problem yourself or let a great piece of software like CleanMyMac 3 do it for you, just make sure you clean up your scratch disk. Your Mac will thank you for it*.

*Not literally. That would be weird.

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