I've tried to export from Photos to hard drive but it says my Mac Hard Drive is in "read" only mode How change so I can export? Took me forever how to finally figure out how to export photos/videos from the new Photos app to an external hard drive - which I must do to free up space as I have over 100 gig of footage I've shot of my 100 year old grandma - with more to shoot.

It says my Macintosh Hard Drive is in 'read' only mode so it won't let me export.

Can you tell me how to change that so it can allow me to export and I can move these asap?
If you are running OS X 10.10 "Yosemite" or earlier, see this Apple support article.