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So far as I know, the magnetic domains in HD's are permanent, or as near as makes no difference. Once stored, data resides there forever. SSD's, as I understand it, decay. The charges used to store bits die away and the bits are lost. Data on SSD's is temporary.
Or am I out of date with the current state of the art?
I know the mechanics of HD's don't last forever if they are used, but the data doesn't decay if they are taken offline. Is that right?