[CentOS] UEFI booting

Thu Sep 19 21:21:33 UTC 2019
Jonathan Billings <billings at negate.org>

On Sep 19, 2019, at 7:42 AM, Jerry Geis <jerry.geis at gmail.com> wrote:
> I installed my first UEFI disk yesterday. Seemed to go fine. CentOS 7.6
> x86_64
> I then took that disk "out" of that machine and put it another machine - it
> seems to not even boot.
> I put the original disk back in that machine and it boots fine.
> I put the UEFI disk back in the machine I built it on and it works fine.
> They are similar machines either and i3 and i7.
> Shouldn't that work?  Build a UEFI disk on machine A - move it to machine B?

UEFI isn’t really a *disk* but a hardware firmware setting on the computer’s motherboard.  Instead of looking for a boot configuration in the Master Boot Record of the disk, the UEFI firmware looks for it on a specially labelled FAT32 partition with an EFI directory on it.  

If your motherboard’s firmware can boot UEFI, then it can boot off the UEFI executables on that disk.  If the motherboard is too old and only supports MBR (legacy) boot mode, it’ll look for the MBR on the disk.  If you’ve got a newer motherboard, but it’s set to boot in legacy mode (which is just a specialized UEFI boot service that emulates the old boot method) it’ll only look for the legacy boot bits.

I would not be surprised that your other system would not immediately detect a UEFI disk if it was set to look for the legacy boot method.

Jonathan Billings <billings at negate.org>