I cannot transfer photos from iphoto onto an external hard drive

My Iphoto library wont transfer to external hard drive

I purchased a Mac 5k Retina 27" desktop and Im trying to transfer my photos from my Macbook Pro (2.5 years old) from Iphoto onto an external hard drive, by dragging and dropping and tried from the iphoto Library folder as well but a message pops up and says it cannot transfer etc etc.

There has to be a way to transfer just my iphotos (19,000 of them lol ) from my macbook - to my new desktop.

Please help!