Hello guys! First of all, please excuse my english, this is a french guy speaking to you... Here is my problem: I upgraded my Mac to Yosemite. Then I created a new admin user, and I deleted the previous one, the thing is that I also deleted the "main folder". After that, when I started my laptop, I had a folder with question mark flashing. I reinstalled OS X from internet, but I wasn't able to find the Macintosh HD disk! So I installed it on a external hard drive. Now I can access my computer, but I still boot on the external hard drive and I am not able anymore to find the Macintosh HD! We tried many things, boot with cmd + s, cmd + r, option, fsck...Nothing works! Can you please help me, this is very problematic...Thank you soo much!