I was wondering.
I normally boot debian from internel m2 ssd. Grub is installed there too.
Sometimes I want to boot to Windows 11. Now Windows is installed in the same ssd.
Would it be possible to install Windows on a fast usb drive, and make an option in Grub2 (ssd) to boot Windows 11 from this usb drive? I suspect that would not be a problem, but what if I remove this usb drive? Is it possible to make this Windows entry optional?

Possible, but not that simple as Microsoft has designed windows so that it will not install on an external USB or other drive. This is because microsoft is selling you a license to use its OS on a single machine. If you try with the standard windows installer you will see a message indicating it will not install to an external device. There is software designed to do this and the page at the link below explains it in detail along with the limitations. Haven't used this software myself so don't know how well it works.

https://www.tomshardware.com/how-to/...indows-11-disk

You should be able to update grub with the external device attache to get a windows entry or simply add an entry to the grub.cfg file.

If it is not set as the default, it will always be optional as the entry will appear and you select or not select it.

Thank you Yancek, I will give that a try then, as soon as I get my new laptop. I did not know that about grub. I thought I remembered that grub ended with an error when the partition of an entry is missing. But hope you are right.

That will happen if you select the option (windows) when the drive is not attached. If you update grub when the drive is not attached, it will obviously no longer show. If you create the entry manually, you should have it (windows menuentey) in the 40_custom file in /etc/grub.d rather than the grub.cfg file as it will not be included when grub is updated..