Ancient slow, cylindrical magnetic media that were once state-of-the-art storage devices. Under BSD Unix the disk partition used for swapping is still called "/dev/drum"; this has led to considerable humour and not a few straight-faced but utterly bogus "explanations" getting foisted on newbies.
See also "The Story of Mel".
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Nearby terms: DRUCO I « drugged « drug report « drum » drunk mouse syndrome » dry run » DS0