DNA precipitation?

[daniel morris]

My guess is that the flakes are salt flakes which have come out of
solution.  In general, I would suggest not performing -20°C pptns:  5' on
ice is more than enough, and from what I have read, as long as your DNA
concentration is greater than 20ng/µl (or so) NO incubation is required. 
Saves a lot of time ! (and in fact you can DECREASE your final yeild using
-20/-70°C incubations as a consequence of the solution viscosity
increasing).  Hope this helps, Dan.