Creating a bootable (non-UEFI) backup copy of a bootable (non-UEFI) Debian hard disk

[local10 <local10@tutanota.com>]

Hi,

Am looking for an (easy) way to create a backup copy of working bootable Debian Buster HD. The idea here is to have a second bootable fully-functional HD which is normally offline but updated from time to time, including all user data and installed packages.

So for that purpose I have two HDs (working and backup), both are the same model, the same size, both are partitioned exactly the same way (three partitions: /boot, /, swap). Rsync-ing "/" partitions is not a problem and coping boot partition files from "working /boot" to "backup /boot" could be easily done too. However, I'm having issues making the backup HD bootable. Either I'm doing something wrong with grub-install and/or perhaps just coping files from "working /boot" to "backup /boot" isn't a good idea?

Thanks