You seem to imply in your response that there is an iPhoto Library and a Photos Library. Is that correct? If I have been using Photos exclusively since the "upgrade" from iPhoto, is it safe to delete iPhoto and it's associated Library? As you can see from my earlier post above, I may have been led to believe that iPhoto and Photos shared the Library and it was not safe to delete iPhoto or it's Library.
Visually, there are supposed to be two separate libraries on your hard drive, one for iPhoto and one for Photos. Physically or literally, it is a bit more confusing (at least for me). Based on everything that I have read since last spring (which is quite a bit), the Photos Library isn't a literal duplicate of the iPhoto Library. Instead, it behaves something like an alias of the iPhoto Library, but it contains "hard links," not just pseudo aliases (if I am remembering correctly). It may be sort of like the concept behind Time Machine's file structure, where there are many virtual copies of the same files, all of which can stand on their own if necessary. Many people have said they have safely thrown away their iPhoto Library once they created their Photos Library. They may be right about it being safe, but I'm not very trusting (of my own one-in-a-million bad luck, mostly). Having two libraries of this type is not supposed to take up double the space on your hard drive, but some are convinced that having two of them does take up extra space; therefore, they have thrown away whichever library they don't intend to use anymore. Most didn't encounter problems, but some may have, if I remember correctly.
If one ever feels a need to revert to iPhoto, it would be very unfortunate if their original iPhoto Library no longer exists.
You could try this quick experiment:
1.) Create a temporary "iPhoto Library Test" and put 100 or 200 photos in it (duplicates of the originals, of course).
2.) Do two or three minor edits and create an album or two, just to say you did some customization of the library (probably not necessary, but...).
3.) Quit iPhoto.
4.) Open Photos with the option key held down on the keyboard and open "iPhoto Library Test."
5.) Do two or three minor edits in Photos.
6.) Quit Photos.
7.) Throw away the original "iPhoto Library Test" and empty the trash.
8.) Reopen Photos and see if "Photos Library Test" and all the photos in it are still really "there." If they are, then you have your answer.
