Q: can't export images from Photos

In the Finder, please press the key combination shift-command-C, or select

          Go ▹ Computer

from the menu bar. A window will open showing all mounted volumes. Select the one in question and open the Info window. What is shown as the Format in the General section?

Are you running a Managed or Referenced library?  Check the General Preference pane for Photos and compare to this screenshot:

Which is it?

OS X El Capitan: If you don’t have permission to use files on a disk

Please click the link and follow the instructions.

Use the Photos library repair tool

Only some images fail to export? Not all?

If repairing the library didn't work, you must be missing some data. The only way to solve that problem is to restore from a backup that predates it, if you have one.

"The export operation failed to create a file for the image identified below: IMG1234 Could not load image".

There are two possible reasons for this error message. Either Photos has lost the connection to the original image file, but the file is still in the Masters folder inside the library, or it is indeed missing as Linc Davis suspects.

Repairing the Library cannot always fix a broken connection to managed original image files, even if the permissions and access rights on the drive have been repaired. It is not implemented well in the Library repair. On the slim chance, that the originals are still there inside the Photos Library, but not correctly connected, do the following:

• Select your Photos Library in the Finder.
• Ctrl-click or right click it.
• Select "Show Package Contents" from the contextual menu. This will show you the inside of the Photos library.
• In the window that will open, select the Masters folder.
• Now search for the missing image file by typing "IMG1234" into the search field (any filename you have seen in the error message).  Set the search range to "Masters".
• 

Does this search find your missing original file?

Then copy it out of the Photos library and try to open it in Preview.   I find it suspicious, that the filename extension is not shown in the error message.  The file may be in a file format, that iPhoto could open, but Photos cannot handle, for example the PCT format.  Try to find out, what the file format of the problematic files is.