grub2 - Grub transfer to another boot device

This is the story:

I have installed Xubuntu 18.04.1 on an external USB HDD. After the installation, I discovered that the operating system was booting ok after every system restart but it wouldn't boot after a system power on.

When I pressed the system's boot menu button I discovered that the USB HDD was not detected by the BIOS during a system power on but it was always detected after a reboot. I realized that this was happening because the HDD would not spin up in time for the BIOS to detect it when the system was powered on.

That issue was forcing me to press Ctrl-Alt-Del after every system power-on so that the BIOS would detect the external HDD after that, since the spin up procedure would be complete at that point.

In order to resolve this issue and since there is no internal HDD on that laptop, I decided to use the laptop's card reader and create a bootable micro-SD card so that the system would boot from that card instead and switch to the HDD afterwards. The grub boot delay helps a lot and the USB HDD completes the spin up procedure in time now. On the running system I executed grub-install /dev/sdb and update-grub and grub did get installed and updated on that micro-SD card successfully and the system boots successfully every time now.

The questions are:

1. Did grub-install /dev/sdb install grub on the micro-SD card only or did it erase the USB HDD's (/dev/sda) grub too?


2. If not, does update-grub update both grub installations every time it's executed from now on?


3. How do I disable or erase the USB HDD's grub so that the system boots from the micro-SD card exclusively?

Thank you.