I don't have an active problem with this but I am curious.
Over time, my repair work has lead me to experience many instances (repeatable and reliable ones) where a computer (desktop, laptop, every situation is unique but is consistent in itself) will have problems during the POST/boot phase of startup. An example:
One computer that I have will hang during boot (I forget exactly where this is, but it's after POST) when I have an external USB storage media in it. This can be hard drives, or USB drives. It's not EVERY USB HDD/Flash stick, but it is a significant number. So much so that when I boot, I try to make sure it doesn't have these devices plugged in. I can't remember if it matters WHICH USB ports are being used, but I know that I typically use the front ones.
Another example was a laptop that I worked on, which hung after POST when an Ubuntu LiveUSB was plugged in, unless I used a specific port, which was almost completely destroyed inside.
A third example would be another desktop machine which, when booting while a USB hard drive is plugged in, hangs during boot at the point where you can see text on the screen. During the pre-OS boot phase. If I unplug the hard drive, the boot immediately resumes as if there were no problem.
Why would external devices (some bootable, some non-bootable) cause this kind of boot issue? It's wide-spread, and non-uniform, and I'm curious what happens at a machine level.