You should be able to just drag and drop the library from the EHD to the Pictures folder on your Mac.
You can try using a copy/backup/clone app to copy library.
Another method which has worked for some in the past is to create a new folder in your Pictures folder. Open the library's package on the EJD and copy the components inside the library to the new folder. When all is copies name the folder Photos Library.photoslibrary and try opening it with Photos
Question regarding the empty squares: are these the thumbnail version or the full version (double click on the thumbnail to open)? If they are the thumbnail version try rotating them one direction and then back again to rebuild the thumbnail.