Problem merging two photo libraries - help!

Hi - the work migrate is confusing me. Normally I've seen migrate used by photos to refer to migrating from an old iPhoto library to the photos library - but you are also showing iCloud activity which can't happen until after the library has been converted. Can you confirm that this is already a Photos library, not an iPhoto library?

What format is the disk "Photo back up"? How much free space is on it. You may need double the space of the total final library, since much of the iCloud library must be downloaded for the comparison of libraries.

Also photos is very particular about the format of the drive the library is stored on:

It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives) and not case sensitive.

It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)

It must be able to have the 'ignore ownership" checkbox ticked.

It should not be on a memory stick, SD or similar.

It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt). File sharing services such as iCloud Drive, Drop box, Onedrive etc are not compatible with Photos libraries.

See

Move your Photos library to save space on your Mac – Apple Support (UK)

Hi

This is actually quite easy - and you can merge the two libraries using iCloud. (As keith has said, you can't easily merge them offline unless you have Monterey)

If I understand correctly your smaller library is already synced with iCloud.

If that is correct, all you need to do is open the larger library, set that to be the system library, check that iCloud is still enabled, and then it will sync with icloud - uploading what is in it, and downloading what is already in iCloud.

So open the large library, set it to be the system library (in preferences, click the circled button)

Then go to the iCloud tab and check that iCloud photos is checked. You will then need to wait - with 400+GB it is going to take a while - days or even a week or two. Leave the mac on 24/7 photos open, sleep disabled, and connected to power. Be prepared for the status indication not to show any progress for long periods. Also during this time, it is likely new photos from your phone won't transfer when you want them to. You may want to use a USB connectdion and the image capture app until the sync is complete.

No if it is used for time machine you can't put a photos library there (Permissions structure needed for time machine is not compatible with Photos library). Also not if it is network connected (WIFI or ethernet)

What operating system are you on? Merging Photos libraries requires Monterey, unless you have a third party solution.