Comment 84 for bug 1396379

Tim, I'll refer you to my earlier response to you on this exact same question 1 year ago: https://bugs.launchpad.net/ubuntu/+source/grub-efi-amd64-signed/+bug/1396379/comments/61

This bug DOES clobber boot setups.

In my specific example, I was trying to leave the laptop untouched. The Windows bootloader would remain untouched on the laptop built-in storage. I would change BIOS settings to prefer USB boot, and install Ubuntu to a USB stick. The GRUB setup on the USB stick would ONLY have Ubuntu configured. Intended end result: boot with USB plugged in, system acts like it only has Ubuntu and boots immediately into Ubuntu. Boot without USB: system acts like it only has Windows installed and boots directly to Windows with no delay.

Actual result: Windows bootloader overwritten with a grub config containing only Ubuntu. Windows is completely unable to boot. Ubuntu is completely unable to boot unless the USB is plugged in.

I see no way to spin this as not clobbering data.

I was able to recover in the end, I can't remember how, probably I used Windows repair on bootable Windows install media. Years later, now, the laptop actually has Xubuntu installed directly with no trace of Windows at all. But at the time it was infuriating. I "ruined" my kid's laptop trying to do things in a way I perceived as minimal risk, double- and triple-checking the installer settings to make absolutely certain I wasn't touching the main laptop storage.