I have run out of space on my 11.6" MBA. I only have iPhoto library of 10GB except that i don't have anything on this mac, Still i have 0 MB left on my 64GB SSD. The mac has really slowed down since past couple of months. I am currently on Yosemite.. So i want to know how can I reinstall Yosemite ?? Also how can I take a backup of iPhoto library ?? Just copying the photos on a External drive will do ??