By default, the GRUB boot loader is installed on the first disk in the system, it is usually a hard disk, but on some servers with RAID controllers I noticed that when you turn on the server, select the USB flash drive in the Boot menu, of course, to boot from it, it becomes the first disk, then there is /dev/sda and at the end of the system installation the bootloader is installed on it.
The solution to this problem is simple – at the end of the installation, we will specify manually where to install the bootloader, the hard disk is usually in this case /dev/sdb.
In Ubuntu, for example, to the question “Install the GRUB bootloader in the main boot area?” answer NO and specify /dev/sdb.
See also my article:
Recovering GRUB Linux