How do I import photos in to the Photos Library completely? I get the following error in Photos app on my Mac.

Missing File
Photos with unavailable original files cannot be opened.
The original photo “IMG_3076.JPG” is either offline or cannot be found. Click “Find Original” to reconnect.


This is what has happened, I assumed that since the photo is in the Photos Library I could delete the file from the hard drive. Apparently not. The answer by @Tetsujin suggests that this is not the case.


I have a backup of all these files and I can restore these. 
My question is, How do I import these files into the Photo Library completely, so that there is no longer any need to keep the file as well?
Restoring them to their original location is the best plan. Then use File > Consolidate (as suggested by @Tetsujin here) to ensure all items are copied into the library. Your settings (as shown in the screenshot in your other question) are set correctly for Photos to manage the library and the originals to be able to be removed from your hard drive. This time I would just rename the folder they're in and see what happens. That way you can quickly and easily undo it if it still isn't working correctly.

Unless you are referring to the Masters folder (within the .photoslibrary bundle) as the originals. In that case - leave the photoslibrary bundle alone. You cannot remove files from it and expect them to keep working in Photos.