I booted from external and wiped my internal HD on my iMac (late 2k9 3.06ghz, 8gb, 500gb). Setup El Capitan, fully updated. Everything works well, other than Photos. I have 6k photos in the cloud. Setup Photos and let it load everything, clicked download originals to my iMac. Closed it and reopened with Command-Option and rebuilt the database. It says in preferences that iCloud that the computer has backed up and is fully complete. After 2 attempts to rebuild the database My issue remains. I can open any photo up to 3/4 of the way into my list of photos. The last 1/4 or so just throws the exclamation point error when expanding it from thumbnail, leaving my remaining 1500 or so photos inaccessible.