There are 224'575 images in my library. Everytime when I scroll down the list I see the red photos which are displayed when a thumnail is not available.
When I start Tools\Options\Build missing thumbnails MC sometimes reads ae few hundred or thousands, sometimes it tells 'complete'. In that case, I restart MC, scroll down again, see some red photos and call Build missing thumbnails again. Then again hundred or thousand images were thumbnailed.
I repeated this at least 15 times - and it still has not built all thumbnails.
It seems to my, that only the images are thumbnailed which have been displayed while scrolling.
Also I saw several times that 'Building thumbnails' didn't finish - sometimes it just finished without reaching the calculated count, sometimes it just stopped or became awfully slow.. In that situation I tested the availability of all my drives with explorer. They all were responding. After restarting MC it worked well afain - at least for a while.