Hi Leonard,
1) it seems that the thumbnails which were created at some point in time remain stable and do not disappear. The others that were never generated, remain as placeholders. It seems totally random which one is not generated.
The little clock icon in the status bar indicates e.g. 30000 thumbnail updates remaining. [30000 is just a sample. Each time Photo gallery is opened and a folder is clicked it changes.]
The number depends on how many folders I clicked before. However, it seems that this number then is not updated at all, so no updates are done.
2) all are jpg
3) it happens to "My Picures" folder on drive c: and also to the images located on my external USB 3.0 drive. It makes no difference where they are located.
4) All pictures come from cameras, but no specific brand or model. It happens to images from 5 year old cameras and to newest camera images. Altogether may be 10 different cameras and no pattern is recognizable.
MORE INFO:
most thumbnails that were generated some time in the past... are blurred. That type of blur which could be seen always for Live Photo Gallery when the thumbnail was generated BUT then it sharpened a second later. This does not happen any more.
I have this issue since I installed Win 8 preview. In general it seems that the thumbnail generating process which runs in the background somehow 'hangs'.
I have about 90k+ images and they were all displayed perfectly under WIN 7 -- I am aware it takes some time to generate all thumbnails but here under WIN 8 definitely nothing progresses. No restart of Photo Gallery, no boot, no nothing helps.
I deleted already the .pd6 file and it Photo Gallery started to recreate it but stops at a certain point in time. The file does not grow anymore in size, however, the date modified is always updated when Photo Gallery is accessed.
The 2nd issue I described [crash when opening the context menu for an item and clicking COPY] happens for any item regardless whether a placeholder icon or generated thumbnail.
may be those info help to figure out the issue?
regards
frank