I have a intel dq77mk and for over a year now iv not been able to boot windows if I have my external harddrive plugged in. the drive is fine but the pc wont boot with any external drive in. I did a google search for fixes and tried all the suggestions iv found like usb legacy and boot orders but nothing works. im thinking of getting a new board but would like to try here first.
One additional step that you can try is disabling USB boot at all on BIOS so the system will not consider booting from USB because the reason why you may experience that is because the system detects the drive and checks if is a bootable device, which may cause that it gets stuck during that process. In order for you to do that you need to follow this steps:
Please let me know if the suggested workaround solved your problem.
To better summarize, there are two possible reasons for this behavior:
Hope this helps...