Problem importing large numbers of photos into the Photos app

There is a bug in Photos that will cause Photos to stop the import, if two files in the same import session are having the same filename. That is why it is working to import one folder at a time. But if you are having two files with the same name in different folders, you must not try to import both folders at the same time.

Here's one way to find the duplicate named files:

1 - open the external HD icon in the Finder and use the key combination of ⌘+F to start a search.

2 - select "Kind is image" All

3 - click on the name header to sort by file name.

4 - scroll down list looking for duplicate names.

5 - rename one of any duplicate names by adding an "a" to the end of the file name.

Although a bit time consuming it would be quicker than any other way I can think of.

There is an alternative way - if you don't need to also import the folder structure into Photos.

If, in finder, you do a search (Like OT has suggested), but then instead of manually checking, you select them all (CTRL-a), copy, then paste into a new empty folder. Finder will automatically rename duplicate names.

The downside is all your images are now in a single folder.

If that is no good - you could use this copy of the files to identify duplicate filenames - they are renamed by adding a sequential number before the .jpg (or other extension). So you could search for 1.jpg which would identify a duplicated filename, and then scroll down the search list to go directly to that item in the search list.

Here is a screenshot of the test I did - 3 files, each copied into 4 sub folders of the folder "F1". I searched for them all in the F1 folder, then copied them to a destination folder.

This is a shot of the original filenames:

This is how they've been renamed.