Photos - steps to take before deleting a photo folder that exists outside of Photos App library

Photos App v. 5.0.

My Photos "library" is stored on an external HD. Also, on the external HD is a standard folder with 500 photos (500 Photo Folder). I have no recollection of why photos exist in this folder. I'd like to delete this folder after ensuring all photos are safely inside the Library.

I can view the photos in the Photos App. As part of the checking process, I started Photos > Import of the 500 Photo Folder and Photos App showed that no new photos were found so I cancelled the Import. By way of background, the "copy items to the Photos library" is selected.

My basic understanding is that "referenced" photos are ones outside of the Library and all others showing in the Photos App catalogue system are within it. To see if the 500 Photo Folder photos are "referenced" rather than "copied to" the Photos library I created a Smart Album selecting "Photos - referenced" and there were zero results. I also selected all photos in the Photo App and tried File > Consolidate - the app says that there are no files to consolidate as they are all in the Library.

Searching the img file (e.g. "IMG[###].jpeg") in Finder only shows the photos in the 500 Photo Folder. I assume this is because the file name within Library is different because, when I looked within the Library folder by viewing Package Contents, the file names for photos were long strings of letters and numbers.

I'd appreciate help from a more experienced user as to whether I can safely delete the 500 Photo Folder. Also, if there are any other steps I should take to ensure that I am not accidentally deleting the only files of these photos.

Everything I have done so far suggests that the 500 or so individual photos exist on my external HD both within the Library and also (by duplicate?) outside it in the 500 Photo Folder. The Photo Application appears to be handling the photos within the Library rather than accessing them as "referenced" files outside the Library.

Many thanks.