Currently, my computer boots between Windows 11 and Ubuntu Linux. Each installation is on their own drive. I choose which OS to use with the Grub menu.

I want to erase the entire Windows drive and reinstall Windows 11 fresh. I'm sure that this will mess up my dual booting system somehow. If it would make things easier, I would consider wiping the Linux drive as well and reinstalling. All my data will be backed up on a third HDD.

This is an EFI system, so I also need to worry about where the EFI partition lives.

Normally, I would just disconnect the Linux drive and do the installation, but it's a M.2 drive that would require a lot of disassembly to remove.

How can I be sure that grub is currently installed on the Linux hard drive and not the Windows drive?

Any advice before I start breaking things?

Why must you erase the Windows drive? Just tell Windows' installer to replace the existing Windows installation. It won't have any reason to change any partitioning if you choose to install in place of the existing Windows installation. With UEFI, there's nothing Grub put on disk for Windows to want to overwrite. The most that would need to be done is to re-prioritize within BIOS setup, or using efibootmgr.

Be sure to designate formatting only the Windows system partition if you have only a single ESP that is currently shared by Windows and Ubuntu (which is not what your description suggests to be your current configuration).

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Thanks for the help! I had forgotten about the Window "reset" feature. That should be more than enough.

After reinstalling Windows, you will likely need to update grub, as Windows tend to ignore any bootloader but its own.

This looks like a pretty good tutorial, but I have no way of testing it. A web search for "update-grub windows" will turn up others.

So I did the Windows reset, clearing all my user files and apps. It went well and I did not have to fix anything as far as dual booting. Grub still works.

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A similar option when installing windows is to select the 'Custom' install option on the windows installer. You should see this immediately after the EULA page at the beginning of the installation. The reset option seems best in your situation and the 'Custom' option would be used to select a windows partition or unallocated space on which to install so a user does not overwrite an existing data or system partition.