Scratch monkey is a term used in hacker jargon, as in "Before testing or reconfiguring, always mount a scratch monkey", a proverb used to advise caution when dealing with irreplaceable data or devices. It is used to refer to any temporary configuration changes to a computer during any risky operation, which include a replacement for some precious resource or data that might otherwise be destroyed.
The meaning is based upon the use in the 1970s of a scratch tape or other storage device, which was available for temporary use, to be loaded in place of other tapes whose valuable contents might be damaged by the operation to be performed. If a problem occurred, it would be the scratch tape that was damaged rather than the more valuable tape that had been removed.
The phrase "always mount a scratch monkey" originated from two tales by technicians about maintenance that was performed on computer equipment. The technicians were unaware that the computer was connected to five laboratory monkeys, and the routine maintenance procedures caused the death of three of the monkeys.