Photos not uploading to iCloud

OK - I can help with that....

1 - Create a new EMPTY library in your pictures folder if there is enough space on system drive - otherwise on external drive. (Hold down the option key while starting photos, and click "create new". Name it something that is easy to recognise).

2 - Open the new empty library in photos. (it should open after you click "create new" but if not, use the option key start again, and select the new library from the list.)

3 - In photos preferences set it to the system library (click the circled button)

4 - In photos preferences iCloud - turn iCloud on with download originals.

5 - Wait - for photos to download all your images from iCloud into your freshly minted library. It may take some time - possibly days. Leave the mac on with photos open and sleep disabled while it is doing it.

HI

On the old iMac, did you have "optimise mac storage" selected? If so the library you restored from backup won't contain the original photos.

Are all your photos in iCloud?

What does the sync status say at the bottom of your all photos view?

As long as a sync is going on, then new photos won't transfer.

The upgrade will have triggered a full new sync of the library. I can't say why it has stalled with all those "unable to upload" images.

The best workaround (since you have everything in iCloud already) might be to sync iCloud with a new empty library - just let it download everything - no need to upload anything. If you want to go that way, I can post steps.

Good news - thanks for letting me know.