Dividing a very large photo library into smaller libraries

Is your Library on an external drive? If so, how is your drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database.

See this:

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

If this drive is in a an incompatible format, stop running Photos with it immediately! An older OS like Monterey will try to open the Library when it shouldn't. A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

I'm not sure what "To" means as a measurement unit. Do you mean terabytes (perhaps 100 thousand pictures,) or something else?

Most of us who handle multiple Libraries use the trusted 3rd party app PowerPhotos ($30.) With PowerPhotos, you can maintain folder and album structure as you copy pictures between Libraries. PowerPhotos uses Photos to do the work, so it is harmless to the Libraries, but it therefore requires properly formatted drives.

Let us know more, and we can help with our experiences…

When I need to spilt a very large library I duplicate it and delete one half of the photos in one library and the other half in the copy of the library.

It helps to do the splitting by using two smart albums, one for the keepers in library one and one for the keepers in library two. For example based on a date rule. Date taken is before January 1, 2000 and date taken is after January 1, 2000.

Duplicating the library avoids the problem with having to import one library into another library and will also preserve the albums and folders.

Marichankobe wrote: The thing is my library is 1 terabytes big, copying it is gonna take forever (assuming that I still have the space on my external drive, which I don’t …)

Since you have an APFS drive, duplicating won't take long at all. APFS is very smart. I'm not sure, but you may be even be able to fit a 1.1 TB library and its copy on a 2 TB APFS drive.

I saw that we are supposed to be able to import photos from another photo library directly so I don’t understand why it’s not working here

Good question.

"I tried to create a new photo library and import some photos from the big library but it's not letting me even access the library by saying that I need to "repair the library"

Which Library are you not allowed to access? If you can't access the 1TB Library, then there's no point in duplicating it. It's possible that the Library is corrupted. You said " copy my library … again" -- is this a copy? Has this copy ever worked?

léonie wrote: When I need to spilt a very large library I duplicate it and delete one half of the photos in one library and the other half in the copy of the library.

I agree-- it's way easier to delete than to copy.