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Scratch space is space on the hard disk drive that is dedicated for only temporary storage. It cannot be used to permanently back up files. Scratch disks can be set to erase all data at regular intervals so that the disk space is left free for future use. The management of scratch disk space is typically dynamic, occurring when needed.
Scratch space is commonly used in graphic design programs, such as Adobe Photoshop. It is used when the program needs more memory, and the amount of available system RAM is insufficient. A common error in that program is "scratch disks full", which occurs when one has left the scratch disks configured to the default setting, being the boot drive. Many computer users gradually fill up their primary hard drive with permanent data, slowly reducing the amount of space the scratch disk may take up.
Partitioning off several gigabytes of hard drive and leaving that space empty will ensure a reliable scratch disk. Hard drive space, on a per-gigabyte basis, is far cheaper than RAM, though performs far slower. Though dedicating a separate physical drive from the main operating system and software can improve performance, a scratch disk often will not match RAM for speed.