A scratch disk is a hard 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 memory or RAM of your machine. 

By default, Photoshop uses the hard drive on which the operating system is installed as the primary scratch disk. 

If you encounter a 'Scratch disk full' error, it often means that your hard drive (or drives) used as scratch disk is (are) running out of storage space required to perform a task.

How to set your scratch disk preferences

You can tweak scratch disk settings in the Preferences > Scratch Disks section.

How much scratch disk space do you need?

Minimum free space on a scratch disk should be 6 GB for Photoshop desktop. However, this is the minimum space that Photoshop tries to keep free in the scratch disk, and the available space that Photoshop considers in a scratch disk is the 6 GB subtracted from the current free space in the disk. So, if the free space in your scratch disk is 10 GB, available space that Photoshop will consider in the scratch disk is 10 - 6 = 4 GB, which may/may not be enough for the current operation being performed. 

• If you make small changes only, you need a minimum of 10 GB scratch disk space for Photoshop (with default preferences, brushes, patterns, etc). It is recommended to have minimum 20 GB of free space on your OS hard drive while working with Photoshop. Additional free space may be required based on the file type you're working with. 
  
• 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. 

Supported drive formats for scratch disks

macOS: macOS extended

Windows: NTFS, exFAT, FAT32

Drives not recommended for scratch disk

• Thumb drives
  
• Any USB-2 drive
• NTFS formatted drives on macOS

Recommended settings for scratch disks

• 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.

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