Finding duplicate videos on external drives on iMac

Unless your Photo Library is stored on the external, the Photos app cannot index or search external volumes to find duplicates. It can only do so from within the loaded photos library.

If your Photos Library is on the external drive, you can load it in Photos by holding the option key before opening the Photos App, then on the "Other Library" button and navigating to the Photos Library.photoslibrary file there.

Then the duplciates it finds after scanning it wil be placed in the Duplicates section under Utilities on the side bar.

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

Depending on the size of your library the search for duplicates can be long. Keep the Photos library open and in the background at night to lessen the time for searching.

If you'd like to be more proactive in duplicate search you can try either of these two apps which I've tested:

you want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons. 

I've run tests on the these two apps with the following results and found them to be safe to use:

PowerPhotos - $40  

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos, both original and edited versions, along with their metadata between libraries.

PhotoSweeper - $15 - Demo version available.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.

Actually PowerPhotos does more that just find duplicates.  It is a way of viewing photos in a library without opening Photos.  And one can use a column mode to view and sort on a number of image metadata and EXIf metadata which Photos can't as well as merging libraries more completely than Photos can. You need the paid version to detect and remove duplicates for both app. It's my preference.

Just some food for thought.

Wally2007 wrote: find duplicate videos on external drives on iMac using the new photo app OS 27 Golden Gate

The most important thing to know, here, is that many 3rd party apps claiming to "clean up your" device may in fact do more damage than good. You should be very careful!

As Phil0124 says, Photos' own Duplicate Finder is excellent. it should alert you if you try to import pictures that are duplicates of ones already there, so you shouldn't be loading duplicates that you are unaware of. 

 Photos must scan through all the pictures in the Library, and it doesn't see this as high priority in light of the battery charge and bandwidth required. So it may take some time for the Duplicates search to complete. The Duplicates folder won't appear until Photos has found some to show you. As with other background operations, it's best to find good wifi, plug the device in, and leave it alone.  If the device thinks that you're busy using Photos then it won't do scanning that might slow you down.

Be careful, though: In Photos on a Mac there is this warning at the bottom of the Duplicates page: 

(This warning appears under an inconspicuous (i) symbol on an iPad or iPhone.)

It's wise to pay attention! Some pictures look alike, but aren't, like this pair:

I would be sad to accidentally delete one of these.

There may be lots of differences even between "exact" duplicates. They may have different metadata, for instance, that you don't expect. I always compare the Info Windows of two pictures, and before merging or deleting I copy data from one and add it to the other. 

The one great thing that merging does is keep the album organization. If one picture is in the Aunt Ethel album, and the other is in the Grand Canyon album, then Merge will delete one and put the other in both albums. 

I don't always agree with Photo's choice of the best picture to keep. Luckily, merging puts the "loser" picture in Recently Deleted, so you may want to scan through those before you allow them to be automatically erased in 30 days. 

After you've ensured that you agree with the choices, you can select them all, or select a swath of them using a shift click, and choose Merge-- you don't have to do it two at a time. You can also just choose 5 pairs, for instance, or any number at a time. Using Command-click, you can choose non-contiguous groups.

As to 3rd Party, as OldToad says, some you can trust are  PowerPhotos ($40) and  PhotoSweeper ($15.) They use Photos own routines to correctly handle Library data. PowerPhotos is great for all sorts of things, but PhotoSweeper is pretty clever at setting levels of similarity.

As always, it's best to have a backup of your Library..