My motherboard is an Asus M4A785D-M Pro for an older-generation AMD Athlon 64 x2 4000+ and this mainboard is non-UEFI. This means that most of the instructions you’ll find on the Internet regarding this are irrelevant at best, because you will have no such option as “Boot from USB” and because everyone else talks about booting using UEFI.
But I suppose you already know that, so to get back to business, this is what you need to know:
To boot from Windows 8.1 to something else, in a non-UEFI environment, you’ll need to remove the USB stick that you are trying to boot to, prior to commanding the reboot.
- Let’s say you just used Unetbootin under Windows 8.1 and are trying to install Ubuntu – or some other OS flavor – alongside Windows 8.1 or for whatever purpose.
- At this point, you have your USB stick with the new OS plugged in into the computer running Windows 8.1.
- Eject the drive.
- Command the reboot.
- As soon as the BIOS POST starts, insert the USB stick back into the computer
- Use whatever key necessary to boot from the stick, if Removable Drive is not your first boot device in BIOS.
In my case, I press F8, because this brings up the boot pop-up menu and, from there, I select the stick. You may already have, by default, removable drive as the first boot device, so this step is optional.
Anyway, steps three and four are the most important ones when it comes to rebooting into something else other than Windows 8, and I have spent two days trying to find some other workaround, to no avail.
P.S. At this point, I suspect Microsoft of messing around with USB partition flags. I mean, I think that issuing a reboot with some USB stick plugged in, renders Windows 8.1 to remove one of the boot-relevant partition flags from the boot-ready partition of USB device, be it active (A) or boot (B), but I’ve yet to prove this and further investigation is needed at this point.