The Jargon File (version 4.4.7, 29 Dec 2003):

scratch monkey
 n.

    As in ?Before testing or reconfiguring, always mount a scratch monkey?, a
    proverb used to advise caution when dealing with irreplaceable data or
    devices. Used to refer to any scratch volume hooked to a computer during
    any risky operation as a replacement for some precious resource or data
    that might otherwise get trashed.

    This term preserves the memory of Mabel, the Swimming Wonder Monkey, star
    of a biological research program at the University of Toronto. Mabel was
    not (so the legend goes) your ordinary monkey; the university had spent
    years teaching her how to swim, breathing through a regulator, in order to
    study the effects of different gas mixtures on her physiology. Mabel
    suffered an untimely demise one day when a DEC field circus engineer
    troubleshooting a crash on the program's VAX inadvertently interfered
    with some custom hardware that was wired to Mabel.

    It is reported that, after calming down an understandably irate customer
    sufficiently to ascertain the facts of the matter, a DEC troubleshooter
    called up the field circus manager responsible and asked him sweetly, ?
    Can you swim?? Not all the consequences to humans were so amusing; the
    sysop of the machine in question was nearly thrown in jail at the behest of
    certain clueless droids at the local ?humane? society. The moral is
    clear: When in doubt, always mount a scratch monkey. [The actual incident
    occured in 1979 or 1980. There is a version of this story, complete with
    reported dialogue between one of the project people and DEC field service,
    that has been circulating on Internet since 1986. It is hilarious and
    mythic, but gets some facts wrong. For example, it reports the machine as a
    PDP-11 and alleges that Mabel's demise occurred when DEC PMed the
    machine. Earlier versions of this entry were based on that story; this one
    has been corrected from an interview with the hapless sysop. ?ESR]


The Free On-line Dictionary of Computing (30 December 2018):

scratch monkey
monkey, scratch

    As in "Before testing or reconfiguring, always mount
   a scratch monkey", a proverb used to advise caution when
   dealing with irreplaceable data or devices.  Used to refer to
   any scratch volume hooked to a computer during any risky
   operation as a replacement for some precious resource or data
   that might otherwise get trashed.

   This term preserves the memory of Mabel, the Swimming Wonder
   Monkey, star of a biological research program at the
   University of Toronto.  Mabel was not (so the legend goes)
   your ordinary monkey; the university had spent years teaching
   her how to swim, breathing through a regulator, in order to
   study the effects of different gas mixtures on her physiology.
   Mabel suffered an untimely demise one day when a DEC engineer
   troubleshooting a crash on the program's VAX inadvertently
   interfered with some custom hardware that was wired to Mabel.

   It is reported that, after calming down an understandably
   irate customer sufficiently to ascertain the facts of the
   matter, a DEC troubleshooter called up the field circus
   manager responsible and asked him sweetly, "Can you swim?"

   Not all the consequences to humans were so amusing; the sysop
   of the machine in question was nearly thrown in jail at the
   behest of certain clueless droids at the local "humane"
   society.  The moral is clear: When in doubt, always mount a
   scratch monkey.

   ESR notes: There is a version of this story, complete with
   reported dialogue between one of the project people and DEC
   field service, that has been circulating on Internet since
   1986.  It is hilarious and mythic, but gets some facts wrong.
   For example, it reports the machine as a PDP-11 and alleges
   that Mabel's demise occurred when DEC PMed the machine.
   Earlier versions of this entry were based on that story; this
   one has been corrected from an interview with the hapless
   sysop.

   A corespondent adds: The details you give are somewhat
   consistent with the version I recall from the Digital "War
   Stories" notesfile, but the name "Mabel" and the swimming bit
   were not mentioned, IIRC.  Also, there's a very detailed
   account (http://mv.com/ipusers/arcade/monkey.htm) that
   claims that three monkies died in the incident, not just one.
   I believe Eric Postpischil wrote the original story at DEC, so
   his coming back with a different version leads me to wonder
   whether there ever was a real Scratch Monkey incident.

   [Jargon File]

   (2004-08-22)