How Long Does Data Last On A Hard Drive?

magnetic domain fields, not charges are used to store data. It should be made clear that all writeable media have a retention problem. Hard drives and optical media both degrade over time. NASA had a problem a few years ago when they pulled out magnetic tapes on which the space program data had been stored, to find that most of them were unreadable. Hard drives have the additional problem of mechanical bearings, electronics, and interfaces which need to work. I think the only solution for data retention long term is continual refresh, rewrite, and media replacement. A better discussion of this problem can be found at:
http://serverfault.com/questions/51851/does-an-unplugged-hard-drive-used-for-data-archival-deteriorate