Stray strong magnets haven't been a problem for magnetic hard disks for 30 years. I don't think the fading of the magnetic domains on the disk is a problem over merely geological time scales either, though I'd be interested in finding out if I'm wrong. The mechanical parts are the big deal, and for recent disks (last 25 years) there are also concerns with onboard Flash or other EEPROM losing state.
Cosmic rays will eventually destroy anything that doesn't have an active repair process, including archival optical media, but the time scales are longer than when the Sun is predicted to swallow the Earth.
Cosmic rays will eventually destroy anything that doesn't have an active repair process, including archival optical media, but the time scales are longer than when the Sun is predicted to swallow the Earth.
See also my comment yesterday: https://news.ycombinator.com/item?id=33116760