Here, I'll list ideas for disasters.
Meteors and asteroids: Planets with weaker atmospheres tend to get pounded by meteors (or "meat ears" as OP2's Savant calls them). And, at least some of you know the backstory of both OP1 and OP2: an asteroid named Vulcan's Hammer crashing into Earth... not a very good thing to happen to a planet.
Seismic: You technically can't call them "earthquakes", unless they're on Earth. Their magnitudes vary greatly, ranging from just a minor tremor to something completely catastrophic. And they do all sorts of fun stuff, like make faults, form volcanoes, cause rockslides, break gas lines and ignite them, etc.
Volcanic eruptions: You should know what they do. They spew lava and ash. The ash cloud a volcano expels can block out the sun for quite a while, and could have an impact on your food supply and maybe a bit on morale.
The Blight: "Nothing could possibly go wro--" - Nguyen
The result of the overconfidence and unfortunate miscalculation by Eden's scientists. It is an artificial micro-organism that breaks apart organic compounds into their base elements. If you don't want to be turned into your own individual pile of goo, I'd stay away from it. Just be careful, it spreads rapidly, especially during the day, and it spreads faster underneath the surface, making mining hazardous. Lava and fumaroles are too hot for the Blight to survive, fortunately. However, Savants seem to have an irresistable attraction to it...
Supernovas/Star death: ... Or, in other words, get your ass out of that galaxy. Or maybe you wouldn't get in this kind of situation if you don't decide to colonize a planet that orbits a red giant... well, I guess I can't blame you if someone figured out how to force a star to die and use it as a weapon against you. Though, these are
extremely rare naturally.
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Also see Weather Ideas for weather-related disasters.)