I have two 'Photos Libraries' what is the best way to merge them please?

Cambs wrote: … migrate the 2025 smaller library into the bigger one, if thats possible,… How do I then get the Photos app location on my MacBook Pro to point to the larger original library

You can just double click on a Library to have Photos display that one. Or you can use the Library Chooser--close Photos and option-click on the Photos icon to see it:

The System Library is the one that other apps like Safari can use, and the System Library is the only Library that can connect to iCloud. You can change the "System Library" designation in Photos' Settings>General.

Here, "Use as Photos System Library" is grayed out, because it already is.

Since you're using iCloud, the best way to merge Libraries is probably to switch the System Library to the other one, and let it merge with iCloud. Photos copies the pictures from this newly connected System Library to iCloud, doing its best to avoid duplicates, Then it will copy all the merged pictures in iCloud Photos to the System Library, again avoiding duplicates. You'll end up with iCloud Photos and your System Library having identical sets of merged pictures. There should be few exact duplicates, but pictures that are similar except with different names or resolutions, etc, may leak through. This merging will take days to or maybe over a week to complete, but it's all automatic and dependable.

Be sure before you start that there's plenty of room on your drive and at iCloud. And you should have backups of your Libraries on separate drives.

2. Could I then get the bigger library moved over to the newer disk that the 2025 library is stored on? What is the best way to approach this?

You can just drag a Library to a new drive to copy it there. Again, you need to pay attention to which Library is the System Library.

3. Then, with the 3.95 TB library, whats the best way to edit it down please?

I'm not sure what that is. You'll have one Library with all your pictures. Save the originals for backup. Find the pictures you don't like and delete them, or at least mark them for deletion. Here is how I mark favorites:

Choosing Favorites using Keywords in Mac … - Apple Community

As one image shows my cloud photos storage is 1TB, so as the cloud doesnt store duplicates i'm guessing that there will be a whole lot of duplication in the 3.95TB library. I'm also unsure if the 3.95 stays backed up on the cloud when a new photos library is begun?

I'm confused by some of that. iCloud is not a backup; iCloud synchronizes devices. You need to keep your own backups.

You can handle whatever duplicates you have with Photos' Duplicate finder, or you can use the trusted 3rd party app PowerPhotos ($40) for more control.

Let us know how it goes…

Cambs wrote:

1. I have been using the smaller 2025 library this year, (I think I started that one after a glitch with the original 3.95TB Library). Both are on separate external hard drives. What I want to do is migrate the 2025 smaller library into the bigger one, if thats possible, I then want to gradually reduce the size of the 3.95 TB Library.

How do I then get the Photos app location on my MacBook Pro to point to the larger original library

You can hold down the option key before opening the Photos App, and it will ask you to select a library or navigate to a library you want to open. You can then look for the Photos Library.photoslibrary file on your external drive.

2. Could I then get the bigger library moved over to the newer disk that the 2025 library is stored on? What is the best way to approach this?

You'll want to do this first. If the newer disk is already formatted as APFS, you can just drag the library file from one drive to the other. It's a single file. Once its copied over, you can then do as described above to point the Photos App the library on the new drive.

3. Then, with the 3.95 TB library, whats the best way to edit it down please? As one image shows my cloud photos storage is 1TB, so as the cloud doesnt store duplicates i'm guessing that there will be a whole lot of duplication in the 3.95TB library. I'm also unsure if the 3.95 stays backed up on the cloud when a new photos library is begun?

Not much option there, than going and manually deleting the duplicates. If you give the Photos app enough time to index, it should be able to identify and sort the duplicates into a Duplicates folder. From there you can manually choose to delete them if you want to.

click here ➜ Remove duplicate photos and videos on Mac - Apple Support

iCloud will sync the system library both ways, so everything or as much of it as can fit in iCloud is uploaded and whatever is on iCloud is downloaded to the Mac. But will do nothing with libraries not set as System Libraries.

You can open other libraries in the Photos App, and they won't sync with iCloud until they are set as the System Library.

click here ➜ Designate a System Photo Library in Photos - Apple Support

The photos on iCloud will remain there unless they are deleted, regardless of what library is loaded into the Photos app.

I just dont know how to begin with this task, can anyone provide me with a plan of how to achieve this please?

If you are using iCloud with the current library, then that's what is getting synced to it, not your 3.75TB library, and it won't unless it set as the System Library.

If you want to use the old one now as the system library then form the newer drive, then:

1. copy it to the new drive,
2. set it as the system library
3. Make sure iCloud Photos is turned on and wait for it to sync.
4. To merge the smaller library with the larger one, you can use the Import feature in the Photos App, and point it to the other Photos Library.photoslibrary file that holds the photos you want to import.
   1. click here ➜ Import photos from another library in Photos on Mac - Apple Support

If you don't want to use iCloud Library then the best way to merge two libraries is with the paid version of PowerPhotos. A comparison between Photos and PowerPhotos regarding merging libraries is as follows:

I ran a number of tests with a 3119 photo, 72 video and 97 keyword Photos library with keywords, captions, keywords and locations.  I merged it into an empty library using Photos and PowerPhotos.  This is what I found was imported by each;

PowerPhotos:           Photos:

Albums***                                           ----- N/A ------

Smart Albums (as regular albums)** ----- N/A ------

Captions                                             Captions

Titles                                                   Titles

Keywords* Keywords (some but not all-see Note *)

Original images                                  Original images

Edited images                                     Edited images

Loctions                                         Locations

Favorites                                             Favorites

Can detect and exclude                      Duplicates

duplicates upon import or not

Neither method could import/merge projects. 

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library. 

NOTES:

*The Photos app only imported 84 keywords out of 97. PowerPhotos merged 147 keywords, some which apparently were in the images but not read by Photos at the time of import or keywords delete along with photos but not purged from the database.

**The original library had 90 Smart Albums. All were merged by PowerPhotos but, as indicated above, the Smart albums were brought over as regular albums. No album of any kind were imported by Photos.

***The original library had 34 regular albums and 1 folder with some nested albums. PowerPhotos merger all of them successfully.

The original library had 72 videos. Photos imported only 57. PowerPhoto got all 72 in its merge.