I'm having the same problem..
I used the thumbdrive install of ubuntu nbr, I played with it, its ok, but now I want to reinstall OSX.
I am using the SAME external dvd drive that i used the first time i installed it, the same boot disc, same usb port and cables, and legacy is turned on, but like the op of this thread, it only goes into linux, it wont even see the external drive to start boot123.
I don't know why it no longer sees the usb dvd drive and it's driving me nuts. I want to sell my 9 since i have a 10v now.
Any help would be appreciated.