Bootloader behavior when 2 OSs are installed on 2 separate storage devices - UEFI

Question

I have a PC with Windows 7 installed on M.2 NVMe SSD (UEFI Boot). There's no way to disable this M.2 SSD in BIOS. MSI AM4 board.

Same PC also has a 500 GB SATA HDD.

I want to install Windows 10 on to the free space on 500 GB HDD. During installation, M.2 SSD will be visible to Windows 10 and will reflect in Install Options screen (Where do you want to install Windows, screen).

Now if I Select free space on 500 GB HDD, will Windows 10 create a new EFI partition on that HDD with it's own bootloader and a separate boot entry, or will it mess with the existing EFI partition on M.2 SSD and modify the existing Windows 7 bootloader on M.2 EFI partition?

I would prefer to keep the 2 OSs strictly different in terms of separate EFI partitions, 1 per disk.

I also want to avoid temporary removal of M.2 SSD as it would require me to also remove graphics card before I can do so as its heat sink overlaps the SSD. The M.2 slot is too close to GPU slot. It means a bit of disassembly of PC components which I want to avoid.

I remember from MBR days that Windows would still modify the bootloader on First Hard disk on SATA 1 port even if you select disk on SATA 2 port for installation. I am not too sure about EFI boot.

Answer 1

It will not mess with the existing Windows 7 boot loader... it may (and likely will) add its bootloader to the EFI partition along side the Windows 7 boot loader.

This is one of the big differences in how EFI works compared to the older "BIOS Boot" method. Instead of writing the bootloader to the boot one and only boot sector of the boot drive, each OS writes their bootloader to separate image files which are stored on the EFI partition. The UEFI bios knows to look for the EFI partition and then read the list of available boot images. Each will show up as a separate item in your UEFI boot options.

Where this gets a little complicated is Window's own bootloader (going back to Windows XP... maybe before on the NT line) is actually advanced enough to support multiple boot options. The information for the multiple options is stored in what MS calles the BCD (boot configuration data). On EFI systems it's stored on the EFI partition in \EFI\Microsoft\Boot\BCD and is shared between all installations that use that EFI partition. You can reasonably expect that the Windows 10 installation will add itself to the BCD as an additional option which will then be presented to you no matter which bootloader image you boot from first.

To avoid that, if you can't disable the M.2 drive, you will need to remove the M.2 drive entirely while installing Windows 10. In which case you will end up with separate EFI partitions on each drive and the UEFI bios will discover both and the bootimages on both and make them all available to select from.

But, to be clear, Windows 7 should remain entirely bootable after Windows 10 is installed either way. In fact, on older MBR systems the Windows Installer would overwrite the bootloader of the other windows installation (if it was a newer version) but, by adding an additional boot option to the BCD (or boot.ini in Windows XP's case), would still give access to both installations.