Q: Re: Why are my RAW images not being read, showing up as black rectangles?

Which camera are you using?  Is your camera on the list of supported cameras? 

Digital camera RAW formats supported by OS X El Capitan - Apple Support

Rest of post (as posted in the iPhoto forum)

I’m having same problem.  When RAW photos are first downloaded from my Canon EOS T3i via Fujifilm SD card, the photos could initially be seen on the window.  But within a few minutes, the black rectangle appeared.  Rectangle covers the top 90% of each new photo downloaded.

And what has changed since this worked

LN

Basic - but have you simply rebooted? amazing what that can resolve sometimes

LN

Good - some people shut down every night so they always have a fresh system

LN