If I start my system with a USB HDD or Flash Drive (thumb stick) attached, GRUB fails to load. Particularly, it just hangs with a blank screen. Normally I would disconnect the drive and reboot, and it works fine, but I'd really like to figure out why this is happening. It's been happening for as long as I can remember (from Ubuntu 10.x to 14.x). I tried looking in dmesg but I didn't expect to find anything there, seems like dmesg would only be able to log if the kernel was running, where GRUB would be responsible for starting the kernel.
I am not trying to boot from these external devices at all, but it is somewhat of a nuisance to have to disconnect any external storage each time, or if someone else uses my computer I have to be mindful to not leave them attached so they can boot to another OS without issues.