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I installed Linux on a USB drive. I made sure to keep the efi partition in the USB drive itself and set the bootloader to install in the USB drive as well. However, every time I start my Computer grub comes up, complains about not being installed and makes itself comfortable on my windows drive.

I don't want Grub to show up on every boot. I only want it to show up when I have my usb stick connected. It's extremely annoying that. Is there a way to keep grub from overwriting my windows bootloader and still be able to boot from usb?

Unfortunately my BIOS is quite old now and doesn't offer a Uefi boot priority (where I could probably choose windows). It only offers a boot device priority where I have set up Extenal drive before internal. But even without the stick connected grub still starts, although it fails to get past the shell.

Thanks in advance!

I managed to get it working thanks to the comments I received. I'll describe the process as soon as I get around to it.

Aram Becker
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    IF UEFI and you have ESP on USB drive, copy both /EFI/Boot & /EFI/ubuntu folders from internal drive's ESP to external drive. Make sure fstab in USB drive then has UUID of ESP on USB drive, so updates are to correct ESP. Once you know you can directly boot USB drive from UEFI, you can delete the folders in the ESP on the internal drive. Please add to these bugs to provide heat. mount correct ESP during install #23 & #26 https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1396379 & https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1173457 – oldfred Apr 24 '20 at 22:14
  • Safest to unplug internal drive when making Full install to USB. That way nothing can go wrong. Make sure your installer drive boots in the same mode your Windows runs in, BIOS or UEFI. If you leave HDD plugged in during install USB will try to use existing EFI partition for booting. – C.S.Cameron Apr 27 '20 at 02:14
  • Thanks to you both. I wrangled a bit with the boot files from EFI similar to the method @oldfred described and now I managed to get it working, although I had to manually load windows from grub console at one point. – Aram Becker Apr 30 '20 at 12:09
  • @C.S.Cameron the problem was that I was working on a Laptop (not my own) and the owner did not want me to open it up. Anyways, for a system that's supposedly as intuitive to install as Ubuntu claims to be, "Remove your primary harddrive" is not an ideal scenario for the end user – Aram Becker Apr 30 '20 at 12:12
  • @Aram Becker: It is safest to remove the hard drive when making a Full install USB, especially when working with UEFI. A line is put in fstab to use the existing /boot/efi.UUID. It is not necessary to remove the HDD if you are careful. see Note 2 on the following: https://askubuntu.com/questions/1217832/how-to-create-a-full-install-of-ubuntu-20-04-to-usb-device-step-by-step – C.S.Cameron Apr 30 '20 at 12:31
  • Most new UEFI systems also have a setting for drives "disabled" which is the same as disconnecting it. But most UEFI forget UEFI boot settings for disconnected drives, but refind Windows entries when drive added. Ubuntu either needs reinstall of grub or use of efibootmgr to add/replace ubuntu boot entry, it that drive disconnected or disabled. – oldfred Apr 30 '20 at 14:26

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