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Here, Energy Saver also has always been set to "never", but since 10.8.5 non-startup HDs go to sleep when not accessed for a while.
Judging by the frequency of reports, it's not an isolated incident...
Consider yourself lucky!
Looks like not letting the display sleep *may* be a solution until Apple fixes this. (Obviously, displays going to sleep should not put HDs to sleep despite a "never" setting.)