Today’s Question: As a follow up to your answer about scratch disks in Photoshop, is there a way to actually know if the scratch disks are even being used?
Tim’s Quick Answer: The only way to determine if Photoshop is making use of scratch disks is to monitor the status bar at the bottom of the interface after setting the popup to “Efficiency”.
More Detail: As noted in yesterday’s answer, Photoshop uses hard drive space to supplement memory (RAM) when there isn’t enough memory. The hard drives used for this purpose are referred to as “scratch disks” in the parlance of Photoshop.
Ideally, Photoshop never needs to use scratch disks, because when scratch disks are used in place of memory performance will generally be greatly diminished. This goal is achieved by ensuring your computer has adequate system memory (RAM). I recommend 32GB as a good balance for memory, though more can certainly be helpful if you work with particularly high-resolution images or need to have multiple memory-intensive applications running at once.
If you want to monitor whether Photoshop is supplementing memory by using scratch disks, you can monitor the Efficiency value. To get started, open an image in Photoshop and look at the status bar at the bottom of the screen. Click the right-pointing chevron (arrow) symbol toward the left side of the status bar, which will bring up a popup where you can select what information is shown to the right of the current zoom setting on the status bar. Select “Efficiency” from that popup.
At this point the status bar will show “Efficiency: 100%” to indicate the current efficiency level. If a number lower than 100% is shown, that indicates that there isn’t enough memory and the scratch disks are being used to supplement that shortage.
If you only rarely see the Efficiency value drop below 100%, such as when you’re working on an unusually large image, it isn’t anything to worry about. But if you’re regularly seeing a value below 100% for Efficiency it is time to either make sure no other applications are running when you’re using Photoshop or to upgrade your computer so you have more available memory. It is also worth confirming that you have allocated enough memory to Photoshop in this case. On the Performance tab of the Preferences dialog I recommend setting the “Let Photoshop Use” slider in the Memory Usage section to about 80%.

