2 Correct answers. If you are going to assign a external scratch drive in Photoshop Mac, it must be formatted as Mac OS Extended (Journaled) or it will not show up in the Photoshop Scratch drive list, even if it is showing up in the Finder or Desktop. If it's already MacOS Extended, run Disk Utility First Aid on it. Try to open Photoshop. While the program attempts to open, press Ctrl + Alt (on Windows) or Cmd + Options (on Mac) to open the scratch disk menu. From there, add another drive or delete temporary files to free up space. Another option is to find an app that allows you to clean up space without having to open Photoshop. Holding CTRL + ALT (Windows) or CMD+OPTION (Mac) when Photoshop is opening will launch the Scratch Disk Preferences Menu. If it is full, clearing space on the drive will help solve the problem. Move files to an external drive, delete unnecessary files, clear the recycle or trash bin on the computer. Enthusiast , Feb 14, 2018. Hi, I you got your scratch disk full, it's cause you hace no or little space on your hard drive. In Photoshop you can set up your scratch disk on the preferences panel, you can choose and external usb disk, if you want. 2 Upvotes. On MacOS: Open Finder and press Command + Shift + G together. A dialogue box will appear. Type /tmp in the text field and press Enter. This will open the temporary files directory. Look for any files that begin with 'Photoshop' ('PST' or 'PSA'), select them, and then delete. 2. Clear Photoshop's Cache. I am trying to change the scratch disk to my external hard drive by going to preferences> Scratch disks etc but my External HD isn't showing as an option and it won't allow me to de-select the Mac HD option either. Even when I launch PS and press Command + option it doesn't show my external HD as an option. 4DDiG Partition Manager. 4DDiG Partition Manager is a simple and secure professional system migration tool that can migrate Windows OS to SSD in one click without reinstalling your system, effectively clone your entire hard drive to backup your data and more. MY WEBSITE: http://www.kingsley.techMY GEAR: wthjVm6.