
If you have a library that has gotten too large and you want to split it into multiple, smaller libraries, the best way to do that currently is to use the "Duplicate Library" command in the File menu to make a copy of the library folder that you want to split. You can then open one copy of the library and delete whatever photos and albums you don't want in that library. Then, open the second library and delete the other portion of the photos from that library. Note that deleting items in Photos will not immediately remove them from your hard drive. If you want to delete the items permanently and free up disk space, open the library in Photos, select File > Show Recently Deleted, then click the "Delete All" button in the upper right corner of the window.