El Capitan came with a new app called ‘Photos.’ The new Photos app imported my old iPhoto library, but it didn’t delete it! They are the same photos, the entire library has been duplicated. The large turquoise section to the left is the new ‘Photos’ library. Look at this screenshot from Disk Inventory X: The pink section surrounded by the yellow line is my old ‘iphoto’ library. I ran Disc Inventory X to see where the excess space was taken.
To run well a mac hard disk needs at least 20% free. I just noticed after El Capitan that I lost a lot of space. My MacBook Air is running a 120GB SSD drive so I don’t want my hard drive full of excess things I don’t need.