USB bootable pauses VM

[jakke]

Hi,
I've got a bit of a head scratcher here...

Running VB 6.1 (latest update).
I wanted to create a USB Windows installation media. The Windows iso comes straight from www.microsoft.com and I used Rufus to put it on both a SanDisk USB-key (16GB) and a WD Passport 500GB USB-HDD.

Both storage devices can boot a newly created Vbox VM but once the Windows logo appears on screen (the blue-green logo on a black background), the VM pauses and I cannot get it to restart. Both USB stick and drive result in the same issue.

Installing the system straight from the iso file, I have a working system in no time, so there's nothing wrong with the original windows iso and I definitely have enough space available on my physical hard drives.
So perhaps Rufus screwed up. I transferred the data from the USB disks to the local virtual drive, ran setup and I reinstalled Windows... no problem, job done.
So the VM is able to boot from the USB, it is able to properly read all the data from the USB-disk through Vbox, so why does my VM get to the paused state and seemingly the only way out of this is to reset the machine and get stuck in that same loop again.

[jakke]

Some more info about my environment:
Host: Win 10 Pro x64 / latest updates
VirtualBox: latest updates / extension pack
CPU: i7-4770K
RAM: 32GB
4 cores + 8GB ram provided for guests, together with a 100GB dynamically assigned vdisk.