> While some photos synced, others didn't
I have occasionally had trouble syncing selected macOS Photos library albums to iPad via USB via Finder (especially after the album was updated on the Mac). No matter what (I tried to omit the album, sync, re-select it and sync but it did not help), hundreds of files failed to sync, or there were blank images in the iPad. Eventually I trashed the 55 GB "iPod Photo Cache" folder inside the Photos library package and let Photos create a new, and after that all images and movies synced OK. But make a backup before doing so, and do not modify the package contents in any other way because you might corrupt it.
> some videos are not being imported. Do you have any tricks as to how you'd go about doing this?
macOS 13.5.1 Ventura Photos.app silently fails to import images or movies from a Finder folder in the following situations, although the files are shown in the import dialog or are deliberately selected there:
1. If some movie has a wrapper or a codec that QuickTime can not play, Photos silently fails to import it and also some random otherwise importable images or movies in the mix.
Examples of such wrappers and codecs are: .avi, H.265 Codec ID hev1, Opus audio. QuickTime Player can not play .avi (even with H.264 video and AAC audio codecs), Codec ID hev1 (hvc1 is OK) or Opus audio which is common in Youtube movies. On the other hand, Photos completely ignores .mkv and .webm movies but imports other files in the mix OK.
2. If movies have (Photos generated) .xmp sidecars, Photos silently fails to import them. BTW, Photos fails to import the .xmp location for images.
I have not yet tested this in Sonoma.
> What's the easiest/shortest way to create a duplicate of the photo library in my second external SSD?
I have used Carbon Copy Cloner to backup Photos library. The 1st sync takes its time, but the subsequent syncs are usually very fast.
