Why was grub installed on the internal drive and not on an external hard drive?

Question

linux

In general, I decided to install linux mint on an external hard drive. I downloaded the iso file, wrote it to a USB flash drive using rufus, booted from it, inserted an external hard drive and started the installation. Finally got to the point of installation. I chose the "other way" item, found my external HDD (1 TB) in the list, created a partition for uefi on it (100 mb fat32), a root partition (60 GB ext4), and allocated everything else to the partition / home (ext4) . At the bottom, I chose grub to be installed on an external HDD, clicked next and the installation went. When the installation was over, the first thing I decided to check was Windows on the internal drive, for this I turned off the computer, took out the external HDD and turned on the computer. I thought that everything would go fine, but grub appeared. I wrote exit, and Windows went to boot.
For some reason, grub was installed on the internal drive where windows was installed, although during installation, as I wrote, I chose an external HDD. In the end, I deleted grub via cmd and cleaned out everything left with bootice ....
But I'm wondering why this happened, why was grub installed on the internal drive, although I specified an external drive during installation?

Answer 1

There must be one ESP (EFI partition) in the system. Therefore, here you can only turn off the internal disk or try, before installing, reset the ESP flag to it. The main thing is not to forget to return it back after installation.