Photos on a 2009 MacBook Pro.
When I try to open some of my photos, a triangle with an exclamation point in the middle appears instead. What does this mean and how can I get around this so that the pictures will open??
Question marked as
It usually means that Photos can't find the original file. Are you running a managed or referenced library?
Change where your files are stored in Photos on Mac - Apple Support
In the preferences, you want "Copy to Library" checked.