Took ten exposures of 20 minutes last night and they all came out beautifully. A considerable time investment on a Saturday night.
I tried to stack them as normal last night in MaxIm. First I did a calibrate on them. The little hourglass popped up to show that it was doing it's thing. A few minutes later, a popup error message said one or more of the images could not be calibrated. I clicked on the ok, cancel button, went back to my folder to try again, only to find it is empty!
I have done an image search of my hard drive, checked every file and even the recycle bin. They're gone.
P***ED off.