Sorry that I'm writing here. I'm totally new in forums.
I was just searching and this post was similar to my problem.
Anyways I hope someone can help me with this issue.
Just upgraded to the imac retina i7 4ghz 4gb video 8gb ram 3tb fusion drive.
I had problems with yosemite. safari , notes, reminder ,terminal didn't launch so I decided to do a clean install with the 10.10.1 version .
I made a bootable usb drive through terminal with my previous imac
Restarted the system I held down alt button then I chose the yosemite drive.
After this I went to disc utility to erase my main hd.Everything went well untill a blackout.
So I started the whole process all over again but when I wanted to erase the main hd it didn't offer me the erase option.i don't know what to do.never ever had such a problem and it's not the first time that I'm doing a clean instal. Help.