Can someone explain how Photos stores its files and how to avoid/delete duplicates?

Thanks for the reply.

I've done most of what you suggested. I moved the iPhotos migrated library file to an external hard drive for safe keeping and trashed and emptied it off my hard drive. It saved a lot, but not the full displayed amount. I hadn't used anything form iPhotos in years.

Duplicate Detective is just a third party software that compares files to find duplicates. It doesn't work inside the Photos library like PhotoSweeper does. I used it on my Master folder, and it found 120GB of duplicate files from when I must have imported pictures and video a second time. I trashed and emptied those files, and Photos seems fine. I kept the oldest files, which were probably the ones Photos was using. Maybe I got lucky?

Previously, I had run Photosweeper on Photos and removed all the duplicates it found inside of Photos. I followed the instructions and let PhotoSweeper moved the duplicate pictures to a temporary folder within Photos and then followed its instructions to delete them. Trash emptied. That got my Photos folder a little smaller.

I had also imported all the remaining pictures outside Photos into Photos. I would run PhotoSweeper after each import to see if those pictures had already been imported. Eventually, I got all my pictures inside Photos, but it seems I am still missing some.

My Photos library is 451 GB. I know some of it is soccer matches on HD that I just need to remove. Each match was too big to fit on a DVD...bigger than 4.7GB. I probably have 100 GB of soccer video that I don't need.

Except for deleting duplicates in the Masters folder, I've done what you suggested and it seems to have worked.

Thanks for the info.

A few years ago, someone posted how to find the Masters folder where iPhotos/Photos keeps all your pictures and video. I pinned it to the side dock. Having been a PC guy for 30 years before getting a Mac, I wanted to be able to find my pictures. All I ever did was copy files from the Masters folders to load onto an external HDD. I started doing that to move large video files for the school's soccer coach. I couldn't get the video to load from Photos for whatever reason, and I found that simply copying the file directly was easier.

You're probably cringing right now, but it's not as bad as it sounds. I have Time Machine backups on multiple disks. I don't know how PhotoSweeper works, but it allows you to move all the duplicate pictures to a folder, and then you manually delete them from Photos. PhotoSweeper works great. On the other hand, using Duplicate Detective on the Masters folder allows you to find duplicates without using Photos. In fact, I wouldn't open Photos when I did it. I haven't noticed any problems with Photos after running Duplicate Detective on the Masters folder. I haven't had any library issues. What I don't understand is how PhotoSweeper missed 120GB of duplicates that Duplicate Detective found in the Masters folder. I went back into Photos and found the same duplicates that DD found, and this was after having run PhotoSweeper several times and finding no more duplicates.

Now that I've used DD, I shouldn't need to do it again. I think I just got lucky somehow. I understand what you're saying about not messing with the Masters folder. I considered going to Google Photos, but I don't like the idea of Google scanning my pictures for faces and marketing. That's why I have to learn what's best for using Photos.

Thanks for your advice (now that I'm following it).