I have a set of 77 files in my result set in a 'Grid' view.
All of the files are unmodified iPhone 4S images taken with the camera (JPGs, not PNGs), with the exception of the filenames.
Task: Some of the files required modification to their orientation (portrait/landscape).
Attempted to do "Open Selection With.." with Preview.app using a multi-select of images. FAILURE. (Single selections work.)
Rather than the above, I simply dragged the multi-selected files to Preview.app on the dock. SUCCESS.
After correcting the orientations of the photos, I then noticed the result sets thumbnails did not update. (Their Quick View images, however, WERE updated.)
I then tried:
Redo-ing the search. Thumbnails still outdated.
I adjusted the size of the thumbnails. When I went to full-size where one image showed per "page", the correct thumbnail finally appeared. Going back to more than one image per page, eventually (yes, eventually), appeared updated.
-the USB flash drive is FAT formatted. Unsure if performance is factor, but I did not see the drive's LED flash showing activity during this time.
-I am doing this piecemeal currently, so I will try to add additional details or patterns as I discover them.