Migrating Photos Between Libraries

the only way to do that is using iCloud Photo Library - Notes on Merging Photos Libraries

LN

The user tip describes how to merge complete libraries. If you only need to move a few photos between libraries, you can do it without iCloud. Do you want to move a few, single photos or merge the complete libraries?

If you only want to move a single photo losslessly, you can tranfer it as a master-version pair with the External Editors extension.

I am using this method occasionally.

• Export the original photo with "File > Export > Export unmodified original" and use the option "Export IPTC as XMP" to save the metadata in a sidecar file.
• Export the edited version as a JPEG in high quality with "File > Export > Export 1 photo"

Now you have a pair of the original Master and an edited version on your Disk and can import them to Photos plus the saved IPTC metadata in the sidecar file.

1. If the original is a RAW file, you can simply import the pair of orginal and edited version together to the other library and Photos will treat it as a RAW&JPEG pair and stack the two photos as a RAW&JPEG pair and you are done. You can easily switch between the edited version and the RAW file in Edit Mode with the command "Image > Use RAW as Original" or "Image > Use JPEG as Original".
2. If the original is not a RAW file, import only the original to the other library, then open the original in Edit Mode and use the External Editors extension (it is available at the AppStore for a small fee). In External Editors click "Choose File" below "Replace with image from disk". In the File Chooser dialog select the JPEG of the edited version and click "Done", then "Save Changes". Now the JPEG will be used as the edited version, but you can still revert to the original. It will be as if you had used an external editor on the photo.

Added: The sidecar files are currently having an annoying bug. The date is transferred correctly, but Photos on High Sierra is reading the time always as 00:00:00.

Therefore I would not use the sidecar file with a RAW&JPEG pair. If you import the photos as a pair (method 1), do not export the sidecar file. The JPEG version will keep all metadata and you can see them in Photos, if you are using the JPEG as the original.

The largest storage plan for iCloud allows 2TG of cloud storage. So iCloud Photo Library could hold a Photos Library close to 2TB. But it will only be feasible to hold an Archive of 2TB in iClod, if all devices syncing with iCloud can afford to hold a large local iCloud Photo Library, von with "optimize storage" enabled, the library will need at least 20% of the size of the library in iCloud as a local storage, probably considerably more. And with an optimized Photos Library you all always need a good W-Fi connection to be able to work with your photos. It will be painfully slow, if all photos are optimized and the Wi-FI is slow.

If your iPhone could hold 32GB of photos locally, I would not let the iCloud Photo Library grow beyond 160GB. If the iPhone can hold 256GB, keep the library size in iCloud below 1TB.

thanks to Léonie for writing it - it is a good, detailed explination

You are welcome

there are lots of good articles by users here - Thematic Index to Photos for Mac Related User Tips:

LN

I just added the procedure for transferring a single master-version pair to the user tips, see: Photos: How to Move a Master-Version Pair between Photos Libraries