Two separate Photos Libraries and iCloud syncing

I have a number of Libraries. The Favorites library is the one that I sync with iCloud. Since that syncs with my iPhone and iPad, new pictures are added to the Favorites library, and then I transfer them to the appropriate "iPictures" Library, edit them, and transfer the favorites back to the Favorites library. I (and many others, here) use  PowerPhotos ($30) to make transferring pictures and keeping track of multiple libraries easier.

If I were you, I would duplicate the current Photos library, name it something I liked, and delete all the pictures that I didn't want on my phone. Then, in Photos>Settings>General on the Mac, I would designate this new library as the "System Library," (the only one that can sync with iCloud) and in Photos>Settings>iCloud I would turn on iCloud and set it to Download Originals. You can move the other, larger library to an external drive that is properly formatted.

Move your Photos library to save space on your Mac - Apple Support

You can double-click on a library file to switch back and forth. The System Library will still be the one that syncs even if you're working in the other library. You can also use PowerPhotos to switch back and forth, or you can option-click on the Photos app to get a list of libraries and switch between them.

I would suggest a small, additional step to this procedure. As the current library used by new-to-apple is already syncing with iCloud, we would have to ensure, that all originals have been downloaded from iCloud before duplicating the library, or the copy on the external drive may be incomplete. Before deleting any photos from the future smaller iCloud Photos library the copy needs to opened and browsed thoroughly. If there are warnings about incomplete items, these should be fixed, before deleting any photos from the iCloud Photos Library.

One way to force a download from iCloud would be to use "File > export > export unmodified originals" for all photos from the library, before copying it. I would save the unmodified originals as a backup on the external drive, just in case there are items with missing originals in the copy.

i am not using "optimize storage" for my iCloud Photos library, but whenever I try to make a copy, Power Photos will point out items with missing originals needing downloading from iCloud. It is a pity that Photos does not have a tool to search for the download status of the items in our Photos Library. Other apps can show us, when an item needs downloading - Books or Music for example, or iCloud Drive.

So I understand that on the iMac you have a fully downloaded and synced Library, and "Just Now" is shown at bottom of the Library view. Then you can just drag this Library to your properly formatted external drive, and you will have two identical copies-- but the external drive will not be the System Library and it won't be connected to iCloud. A computer can have only one System Library, and that will still be in the iMac until you change it. I would open the external drive library in Photos and search around a bit, opening some pictures in Edit mode, and make sure everything is OK (It's always been OK when I've done this.) Then, if you want, you can delete the Library on the iMac, and use the Library on the external drive. You could then designate the external drive Library as the system library. However, while you're playing around with this Library, you might want to keep two copies for awhile. Of course, it would be good to have a backup, perhaps with Time Machine on another external drive.

If you want to export pictures to Finder Folders on the external library, that's OK, but not necessary. It is one way of backing up your pictures. But if you've done edits on pictures, then Export Originals will not show the edits. If you Export Photos, then you will be exporting edited versions for which the edits can never be un-done. The Library has both, but takes up little extra space. So I, for one, do not export originals for backup. However, when I use my Nikon rather than my phone, I maintain those original images in folders before I import them to Photos.

Right, léonie! I left out the "Switch to Download Originals and make sure they're finished downloading step" before duplicating. Drat!

After you switch to Download Originals, you should be sure that, at the bottom of the Library view, it says "Synced Just Now."

There is one problem with this, Richard. The status "Synced just now" does not ensure, that Photos has downloaded all originals, only, that Photos has updated all changes across the devices. I have frequently seen "Synced just now", before all originals have been downloaded. When I turn off the Wi-Fi after seeing "Synced just now", there may still be originals missing, when I try to open a photo.

There is no reliable status indicator, that will ensure, that all originals have been downloaded, at least, I have not yet found one. So we have to test, if all originals are there, after we copy the library to the external drive, before we delete the original of the library. PowerPhotos can check for incomplete items. Sometimes Photos will point out incomplete items when we open the copy in Photos.

léonie wrote:

There is one problem with this, Richard. The status "Synced just now" does not ensure, that Photos has downloaded all originals... PowerPhotos can check for incomplete items.

Drat! Thanks for that warning. And, well, that's one more thing that  PowerPhotos ($30) does that makes it worthwhile.