If it is an original drive it may be suffering 'stiction': intermittent loss of rotation from failed/failing bearing lubrication. The next phase will be screaming (these swans do not sing sweetly before they cash in their chips, to mix metaphors a little), or chilling silence. Count on needing a replacement, soon. If you can induce it to spin continuously just once, get any data that you value off the drive, and then inter it.

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