Make external USB drive bootable
Why does the Ubuntu installer touch my internal HDD when I don't use any of its partitions and when I tell the installer to install GRUB on the external HDD?
I have tried this approach over the years, and this always happens. The latest attempt is with Ubuntu 18.04. I know this sounds like a million-time-asked dup, but I am stuck. The fact that people suggest the VirtualBox work around, or they suggest to disable the internal HDD (which I can't short of unplugging it) indicates to me that this is surprisingly hard to fix.
I have a Thinkpad laptop. BIOS settings allow legacy boot devices. The internal HDD is dual boot Windows and Mint, and it is an UEFI boot method.
I want to install ubuntu on an external SSD drive and have this drive one of the boot options from BIOS. External HDD is a MBR partition style, so traditional boot method is required. The BIOS is set to be ok with this.
If I install onto the external drive via VirtualBox, it works. This means that what I want to do is possible. My internal HDD is completely isolated when I do this.
However, if I do the installation a more traditional way, it always screws up my internal HDD booting. That is, I have use a usb stick to make a live installer and boot from it, the normal way to install Ubuntu, things go wrong.
During the installation process, I can choose where to install grub. I choose the external drive. I would expect this not to touch the internal drive. But after installation, I can no longer boot from the internal drive. grub seems to be missing. I can boot from the external drive.
(I fix it by connecting the external drive and booting into the HDD installation, and using boot repair.)
Why is my internal HDD being changed by the install process when I have followed all advice to leave it untouched?