>Saturday we went out to the Quabbin Reservoir here in MA in >search of Bald Eagles. It was below 20 and very windy off the >water. At least 3 times the camera wouldn't shoot and it read >card err. I have 2 brand new scandisk 32g cards, I swapped >their locatons and all was fine for awhile then another error, > swapped them back and no issues.
Did you mean to say that it was a bit colder than 20F, or that it was -20F? Big difference there.
If it was -20F, chances are the camera was suffering cold symptoms. If it's working consistently fine now, there's probably nothing to worry about.
Card formatting errors creep in with cards that weren't formatted in the camera (e.g., in your PC or Mac instead). If the cards were both most recently formatted in the camera, they're unlikely to both have problems. Which still leaves you with cold, wind and relative humidity and or traces of condensation as the cause of problems.
There's also the matter of battery stability in the cold. If the battery was somewhat low during the problem, there's a good chance that it was contributing to a problem. If the battery was fully charged, it likely had nothing to do with the error.
When I plan to hit a trail or conservation area in subzero weather, I usually acclimatize my camera by leaving it in the trunk of the car, in the camera bag, to slowly cool down to something close to ambient outisde temperature before driving out to the target location. No sudden temperature changes, no condensation problems, no issues with card or lens contacts. When I return home, the camera goes into the bag and the bag goes into the trunk. I bring the camera bag into the house and wait an hour or so before taking out the camera, the hour usually being long enough for the camera and its internals to warm up slowly and prevent sudden condensation. I don't bother with any of that in temperatures hovering around the freezing point or warmer; just for subzero outdoor shooting.
I've found that in deep cold - around -20C/-4F down to around -40C/-40F everything except specially lubricated bodies that have been prepared for such conditions get progressively more sluggish until the shutters won't move, lens apertures lock up and LCD modules crack. In really deep cold that is driven even lower by high wind chill factors, polycarbonate materials exhibit some contraction which can badly damage external body cladding (especially on non-pro bodies). In deep cold too - that is, anything below about -30C/-22F - polycarbonate parts become quite brittle which means that opening one of the external doors to change a battery or an SD card must be a very gentle process to avoid snapping anything off.