Two Macs with same Photos library. How to delete one safely?

To add to Toni's explanation: The Apple support document does not say anywhere, that iCloud Photos is a backup. It says we need to make a local backup and "If you ever lose the files in your Photos library, you can restore them from the Time Machine backup. " That is tongue-in-cheek, as the "files in the Photos Library" are not necessarily all our or photos, only the database files and the currently downloaded photos from iCloud, if we are using iCloud Photos. We can recover the albums and the metadata tags, like titles, but not all photos. The Time Machine backup of an optimised library will only include the photos, that currently are not optimised. When you restore the library from Time Machine and the optimised versions are still in iCloud, Photos can download the originals from iCloud and the restored library will work again like before.

Recovering the albums will work with Photos 5 and 6, as long as the photos are still in iCloud Photos. On older systems versions we cannot even recover the albums completely, because Photos would remove all traces of optimised version from the library, when we open a restored copy of an optimised library, before we even had a chance to enable iCloud Photos for this library and fill the gaps left by the optimised versions in the albums.

As anything we do while working with Photos for Mac or Photos iOS will immediately sync to iCloud, we cannot rely on iCloud Photos as a backup, where we can recover accidentally deleted photos. iCloud Photos is always the same as the mirrored versions of the library on our synced devices.

What a Time Machine backup of an optimised library cannot give you, because the Time Machine version does not hold all original image files:

• If you are having a problem with iCloud and need to reset iCloud Photos by deleting all photos from iCloud Photos, to remove a corrupted image or video, the Time machine backup does not suffice. You need a copy of the full version of your library, without "optimise Storage".
• If there is a problem with your network ad you cannot connect to iCloud for several days and need access to your photos, the Time Machine backup cannot Gove you access to all your photos.
• You cannot keep an archive of older, full versions of your library, so you can recover older photos, that you have deleted a long time ago, and that are no longer in Recently Deleted.

If your library on the mac is optimised, then the original photos are not stored on the mac. So time machine cannot back them up. The library is backed up but only with the preview images (which may not even be full size, and will definitely be lower quality).

If you accidentally delete images, then they are also deleted in iCloud, and you lose the originals.

The apple article neglects to consider this issue.

Hi

On the air, go into photo preferences iCloud tab, and switch off iCloud photos.

Once this is switched off, the link to iCloud is broken and nothing you do on your air will be replicated to iCloud (and your other devices)

Having said that, the safest way to then delete the photos on the air is simply to delete the library. (In fact you could do this without turning off iCloud - so it is a sort of "belt and braces" or "belt and suspenders" if you are USAnian.

To be super super safe - you should really have your iCloud library backed up. You should be doing this anyway - to protect yourself from user error. How to do this with an optimised library? See this user tip from Leonie:

https://discussions.apple.com/docs/DOC-250001961

In summary -

back up your library (if you don't do it now, do it soon)

turn off iCloud photos

delete library.

Job done.

Yes, that is correct. Obviously need to have enough space on your mac for the complete library.

The alternative, is the suggestion in the user tip from Leonie - to have a second user account on the mac with that users system library put on an external drive (formatted MacOS extended (journaled) with the Ignore ownership checkbox checked), with originals downloaded and also synced with iCloud.

Then to back up your iCloud library - you periodically log onto that users account, and allow it to sync it's external library.

You can have your main account with the optimised library back up the external drive to Time Machine.

It is hard to guess from the size of the optimised library, because its size depends on the available storage on your Mac.

Look at the size of the photos library in iCloud. You can see it in the System Preferences > AppleID > iCloud: Manage ....

The size shown for the photos in iCloud is mainly the size of the total of the originals. Add to this roughly 25% for the working copies, like thumbnails, faces, database file for an estimate.

My library is showing 183.3 GB in iCloud, and on my Mac the non-optimised version is showing a size of 223.7GB.

my approach is to keep my iCloud Photos Library small - it has only the photos I need on all devices - my absolute favourites plus the new photos I am currently working on. This makes it possible to avoid "optimise Storage". The archive of all photos is kept on an external volume and I am cloning this regularly.