I have a MacBook Air and I recently tried to install Ubuntu on my external HD. I booted Ubuntu from a flash drive and everything had gone fine. I turned off my computer and when I turned it on again I got a message of error and a "grub rescue" line to enter some code. I rebooted holding the "option" key and I selected the Mac partition and everything is fine with my system, but every time I start my system and I don't hold the "option" key, I get the same error and the grub rescue message. I just want things to go back to normal, as they were before I installed Ubuntu on my external HD (which, btw, presented a message that said there was no boot loader). What do I do?