Storing photo's to an external drive

Quit Photos first.

Drag the Photos library (by default in the Pictures folder on your Mac) to your storage device to create a copy.

Launch Photos with the option key held down to get a list of libraries on your system (probably just two: the one you copied and the copy on the external disk); chose to open the copy on the external disk.

Run some basic tests to assure yourself that the copy worked.

If the one on the external disk is to be the only library going forward, make it your System Photo Library. (That setting is in Photos > Preferences... > General. It's a button with the text "Use as System Photo Library" that goes gray when you click on it.

Enable iCloud Photos if desired.

When you are happy with the new System Photo Library and have it backed up somewhere, it would be safe to remove the original library from the internal disk.

You will need to keep the external disk with the System Photo Library attached to the Mac any time it is powered up and your account is logged in. Background processes need to be able to access it.

Here is a screenshot of what I get, when I try to open the copy of an optimised library. This is from Photos 4 on Mojave macOS 10.14.6: It is not possible to open the library at all, without deleting the optimised versions before proceeding.

I cannot test this currently on the most recent version of Catalina. My only Catalina Mac with an optimised library is in my office at the university, which is currently not accessible because of the lock down. It would be great, if this has been changed in the most recent version of Catalina, but I have not seen any news about a change in the release notes.

The answer to most of your questions depends on the details of how you intend to do it. Would you be storing your entire library on the external drive, dividing it into one library on the internal and one on the external, or just moving some of the photos into Finder folders on the external drive.

If you divide your library, at most one of the two can be your System Photo Library and attached to iCloud Photos. Normally, that would be the library on the internal drive; the library on the external drive would be a stand-alone library. Any photos outside of the System Photo Library, including those in Finder folders, would not be reflected in iCloud Photos.

Having a library enabled for iCloud Photos does not by itself save any space on local disk. If you also enable the Optimize Mac Storage sub option, then Photos may save some space by reducing the resolution of the local copies of some photos when storage on that disk gets low. This is completely automated; there are few ways to affect how many photos are optimized to lower resolution, or which ones.

Calling iCloud Photos a backup is not strictly accurate. The cloud version of the library becomes the master copy, and the local library (or libraries if you have multiple devices) slave to it.

Taking local backups is still a good idea in any of those scenarios. (Be sure to use a different drive for the backups, or a drive failure can lose both copies.) Be aware that local backups can only back up what's local, so optimized photos will have backups that are also reduce in resolution.

I think I've covered all of your questions, if not directly or in order. Feel free to ask more specific questions.

markwmsn wrote: ....

Be aware that local backups can only back up what's local, so optimized photos will have backups that are also reduce in resolution.

It is worse than that, Mark: as soon as you open a restored Photos Library from a backup with optimised versions, the optimised versions will be removed completely. There will be missing items in the albums and projects, the metadata for the optimised versions will be lost. You will not even have the optimised version as a placeholder. We need to make regularly a download of the full library to an external volume to have a backup, preferably on a different Mac or at least from a different user account, so we do not need to turn off iCloud Photos for our main user account.

Thanks for the information, léonie, ghastly as it is. If that happens even with Time Machine restores, that should be reported to Apple as a bug. (It's hard to know where a similar issue with a third-party restore should be reported.)

I have not had occasion to run a test restore of an optimized library. Are you speaking from direct experience?

I do make sure that one of my Macs keeps a full library available for backup purposes. (At least, to the extent possible with no visible indicator of whether the set of full-resolution images is complete.)

I'll change the way I caution other users about the dangers of optimized libraries.

. Are you speaking from direct experience?

Unfortunately, yes. I have tested it several times. When I copy an optimised library to an external volume and open it, Photos will warn me that the "incomplete versions" will be removed.

iPhoto? Or Photos, the app that replaced it 5 years go?

There is a wrinkle with iCloud that I'm not sure of. I'll ask this be moved to the Photos forum.

Launch Photos.

On the top left of the menu bar, not the word 'Photos". Click on it. See the Preferences heading there?

Then that means the current library is the system photo library.