open dolphin/settings/configure dolphin/generel/previews select the previews you want, set the ‘skip previewd for local files above’ to ‘no limit’ and check …
if doesnt work, is your second disk being mounted on boot?
Dolphin uses the kdegraphics-thumbnailer while Thunar uses the dbus service via tumbler installed via File Manager Settings for Thunar. Even tho both are saving the thumbnails in your home directory ~/.cache/thumbnails/ (see the folders in there) each has an unique ID file name, and for the same original image the thumbnail will have a different compression, so it might be because of that Dolhin fails to display thumbnails created wit the other tool. I might be wrong on this. Also, i think the new way to create thumbnails on Dolphin, for image files, to me looks like has a bit of regression …
Edit:
In case you want to try, remove from ~/.config/dolphinrc the entry related to [PreviewSettings] and everything under it, save it, and reopen Dolphin …
so go again to dolphin settings/ previews and modify the skip previews for remote files as you desire … on the kde channel it was mentioned that it is a bug and this should give you previews … if it doesnt work than changing kernels should also help…
For all those who are looking for a solution: Install Thunar (thunar, tumbler, ffmpegthumbnailer) and create the thumbnails with it. They will then also be recognized by Dophin.
Adjust the file size in: /etc/xdg/tumbler/tumbler.rc