Can I Resize A Macintosh Partition In Yosemite

I didn’t partition enough space to transfer all of my files from Macintosh HD over to HD2, so all of my music is still on the Macintosh HD, but I want it on Macintosh HD 2.

I want to delete the Macintosh HD (or merge them?) but I don’t want to delete my music. Is there anything I can do? I have an external hard drive if that helps.