I am trying to install Ubuntu 12.04 over Windows 7 using a USB drive. However, I am not able to install it. Installation breaks saying that "bootloader cannot be installed on 
this device." In the details section, I see the ubiquity message saying that it can't read the partition from dev/sda. I've tried various things, boot-repair one among them (<http://paste.ubuntu.com/6056191/>), but no luck. What am I exactly doing wrong?