I have a bootable harddisk with Windows 10. I added a second harddisk and installed centos 8. Both boot well if they are connected separately. I boot into the disk with centos installed and would like grub2 to offer me the choice between centos and Windows.

From within centos, the centos disk show as /dev/sda and the windows disk as /dev/sdb.

I did not find any clear instructions on how to achieve what I want, and I must admit the I am fairly confused with UEFI, secure boot , keys, CSM ....

BIOS used to be so simple .....

Many thanks,


  • It may be worth noting, that /dev/sda is gpt, while /dev/sdb is mbr. Both have the first partition as boot Commented Nov 1, 2019 at 10:12

1 Answer 1


If your Windows disk uses UEFI native boot style and CentOS uses CSM (= legacy BIOS style), then I'm afraid it's impossible: a BIOS version of GRUB cannot chain to UEFI bootloaders, and vice versa.

When installing a new operating system to a computer that has UEFI with CSM enabled, you select the boot method the new OS will use by selecting which method you use to boot the OS installer. This is not exactly obvious. When booting from optical media, you might see two boot entries referring to the CD/DVD drive: hopefully one has the UEFI prefix and the other hasn't, otherwise it may be impossible to tell them apart without further testing.

To complete the installation of an UEFI bootloader, the installer needs access to UEFI NVRAM variables. When the system is booted in legacy BIOS style, the standard mechanism to access those variables (= UEFI Runtime Services) are not going to be available.

Once you become familiar with UEFI, you will find that it can be even simpler than the traditional BIOS. For one thing, a bootloader can be just a regular file with a specific name, not a hidden piece of code embedded in a specific location on the disk.

Most of the complications come from the CSM. In fact, unless your system has specific bugs in its UEFI support, I might generally recommend disabling the CSM and going full-UEFI if possible.

  • I read somewhere, that refind would be able to chainload an MBR-disk from a gpt-installation, but I finally decided to convert my windows 10 disk to gpt. Now it gets detected by grub2 upon installation of centos 8 (UEFI) on the other disk Commented Nov 1, 2019 at 15:08
  • 1
    rEFInd may be able to do it, depending on the CSM implementation in the system firmware. It is not guaranteed to work in all circumstances. As far as I know, there has been no organized effort to make a list of which UEFI firmware implementations will allow rEFInd to do that, and which ones won't. So, I would not rely on this feature being available without testing first.
    – telcoM
    Commented Nov 2, 2019 at 3:02

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