That's hard to do. You really need to do the power-fail detection on
the AC line, or have some sort of energy storage and a dc-dc
converter, which is expensive. If you simply detect a drop in dc
power, there simply isn't enough margin to reliably write a block.
Years (many years) back, Diablo had a short-lived model (400, IIRC)
that had an interesting twist on this. On a power failure, the
spinning disk (this was in the days of 14" platters, so plenty of
energy) drove the spindle motor as a generator, providing power to
the drive electronics for several seconds before it spun down to
below operating speed.
Of course, that was in the days of thousands of dollars for maybe
20MB of storage....
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