Hi, léonie.
I tried it several times but couldn't get it to work. Although the libraries are there Photos won't display the photos and appears to be empty. To clarify my post after I followed your recommendation Photos did indeed create a new library but after I tried and it didn't work I moved it to my desktop thinking it might be corrupted and tried again. The library was in fact created on the same external SSD drive. I am sorry for not being clear in my post.
I have since then deleted all libraries from my computer. The original library is still preserved outside of my computer in 2 back up RAIDs I have so everything is safe. Since I couldn't get Photos to work I read several threads here on Apple's own support site and decided to install iPhoto. It did open the library and it worked fine allowing me to add photos I had stored in my iPhone.
Now I have run into two problems:
1. I had some problems with videos. Last night my computer (a brand new, 1-month old Mac Pro running OS X El Capitan) and iPhotos refused to import videos from my iPhone. The photos were fine but the videos simply couldn't be downloaded for some reason. This morning I tried again and this time the import seemed to have worked. As I sync the videos with 2 iPads I own (a 1st generation and a 3rd generation iPads) I can't seem to see the videos there under photos. I read one thread where you mentioned to another posters something about a file that needs to be re-written because it seems to be the culprit of the problem. I believe the name of the file is 'com.apple.xpchelper.cache'.
2. I don't know how to migrate my library from iPhoto to Photos. I would like to migrate before I experience problems with iPhoto or before there is a problem with my database since the app is no longer upgraded by Apple. How can I migrate my library to Photos ?
Thank you for your help.