A scratch disk is a disk drive or SSD used for temporary storage while Photoshop is running. Photoshop uses this space to store portions of your documents and their history panel states that don’t fit in the RAM memory. Scratch files go into invisible OS-specified folders, except for non-boot volumes on Windows, at the root directory of the drive. When a non-boot drive is used as a scratch disk, the temporary files are placed in the drive’s root directory.
By default, Photoshop uses the hard drive on which the operating system is installed as the primary scratch disk.
If Photoshop cannot launch because the scratch disk is full, hold down the Cmd + Option keys (Mac) or Ctrl + Alt keys (Windows) during launch to set a new scratch disk.
The scratch space required depends on how you edit and the number of history states in memory.
- If you make small changes only, you need a minimum of 1.5 GB scratch disk space for Photoshop (with default preferences, brushes, patterns, etc.) plus two times the size of all the files you have open at the same time.
- If you make large changes to dense pixel layers (that is, using several filters on complex background images or many edits to large smart objects) you could need as many times the size of the original file as there are history states.
Photoshop always reserves 6 GB of space on boot drives and 1 GB on non-boot drives to avoid causing problems for the operating system by running a drive completely out of space. Having hundreds of patterns or brushes loaded significantly increases the required scratch space for Photoshop to launch.
- For best performance, connect the scratch disks to a compatible port that has the highest bandwidth limit of all the available ports. The bandwidth limits for various ports are as follows:
Thunderbolt = 10GB/sec
eSATA = 600MB/sec
PCIe = 500MB/sec
USB3 = 400MB/sec
- To improve performance, set the scratch disk to a defragmented hard disk that has plenty of unused space and fast read/write speeds. If you have more than one hard drive, you can specify additional scratch disks. Photoshop supports up to 64 exabytes of scratch disk space on up to four volumes. (An exabyte equals 1 billion GB.)
- If your startup disk is a hard disk, as opposed to a solid-state disk (SSD), try using a different hard disk for your primary scratch disk. An SSD, on the other hand, performs well as both the primary startup and scratch disk. In fact, using an SSD is probably better than using a separate hard disk as your primary scratch disk.
- Scratch disks should be on a different drive than any large files you are editing.
- Scratch disks should be on a different drive than the one your operating system uses for virtual memory.
- RAID disks/disk arrays are good choices for dedicated scratch disk volumes.
- Defragment drives with scratch disks regularly.
'Scratch disk full' often means that the drive (or drives) used for scratch disks are running low or out of available space.
Free up additional hard drive space by deleting unnecessary files from the scratch disk, or by moving your files off the scratch disk to another storage location.
By default, only the OS drive is selected as Photoshop’s scratch disk. If you have more than one drive or partition, make sure to select the fastest drives with the most free space as your scratch disk(s). You can also resolve a 'scratch disk full' condition by selecting additional drives for use as scratch disks. See How to adjust scratch disk preferences?
On rare occasions when Photoshop or the system crashes, Photoshop’s preference file can get corrupted and this can affect Photoshop’s handling of scratch disks. Resetting Photoshop’s preferences and then reconfiguring the scratch disk preferences, can resolve 'scratch disk full' errors. See Reset Photoshop preferences.
By default, Photoshop attempts to auto-recover your document in case of an unexpected Photoshop exit such as a crash or computer reboot.
Disabling the Edit (Win) / Photoshop (Mac) > Preferences > File Handling > Automatically Save Recovery Information Every [N Minutes] reduces the size of Photoshop's temporary file (scratch) at the risk of having no auto-recovery.