There are a few additional things you should consider if you’re copying or merging photos into the library you have syncing with iCloud Photos. If iCloud Photos is enabled when you do this, it will immediately start uploading any new photos that you copy into your library up to iCloud. This may not be desirable if you’re copying a large number of photos, especially if you want to double check your copy/merge results before committing to uploading the results to iCloud. Here are a couple techniques worth considering.
- Temporarily turn off iCloud syncing. You can do this by following the directions under “Pause library uploads to iCloud” in this Apple support article. Your current collection of photos will remain in iCloud, but your Mac will stop actively syncing new content while this checkbox is off. You can then performing your copying/merging and double check the results to make sure they’re to your liking. If they are, then go back and unpause syncing, and Photos will upload your newly copied photos to iCloud. Note that it will take some time for it to compare the library content to iCloud again, but it shouldn’t need to re-upload any of the photos that were already in the library.
- Work on a copy of the library instead. Instead of merging directly into the library you have synced with iCloud, create a copy of the library first, either using the Finder or the “Duplicate Library” command in PowerPhotos. This requires having some extra free space, but has the advantage that you can keep your iCloud library syncing while you do your copying/merging with the other copy of the library. If you change your mind or decide you want to redo the merge with different settings, simply delete that copy of your library, and the original library remains unchanged. If you’re happy with the results of the merge, then use the Photos preferences window to first make that library the system photo library, then enable iCloud Photos to have it sync up the new photos with iCloud.
One approach you should be cautious with is merging multiple libraries, including your iCloud library, into a new, empty library. The downside to this is that none of the iCloud-specific syncing information associated with your photos will be copied into the new library. After finishing the merge, if you decide to enable iCloud Photos on the new merged library, Photos may have trouble matching up the photos in the merged library with the corresponding copies that are already in iCloud, resulting in duplicates of some or many of your photos. If you’re performing a merge that includes your iCloud library, it’s recommended that you use one of the two techniques above, as both of those methods will preserve the iCloud syncing information.