[CentOS] Install CentOS 7 on MBR hard disk

[Chris Murphy]

So is the end goal to have dual boot? You want to preserve the existing
Cent OS installation on this drive and also install Cent OS 7?

The biggest problem is the installer is really not very smart when it comes
to this use case. It's friendly for Windows and macOS dual boot, but fairly
well faceplants with dual boot Linux. So invariably you have manual surgery
to do pre and post install, or suffer.

Run 'efibootmgr' by itself, if you get boot entries the system is
definitely UEFI booted. If you get an error, it's legacy/faux-BIOS booted.

Certainly legacy boot is the easiest work around, but it can have an effect
on various things including drive and video modes that might be different
than UEFI booting. e.g. one of my older systems when booting legacy brings
up the SSD in IDE mode not SATA, and the system is slower. And it can only
use discrete GPU, the integrated GPU is unavailable. So I advise testing
before committing to legacy mode.

Also, rare, but not all UEFI systems come with a Compatiblity Support
Module (fake BIOS), in which case you're stuck.



Chris Murphy