Where is it configured which drives get updated when I update the kernel,
as part of a regular upgrade using aptitude.

Presumably it has to install the new kernel, build an initrd, find out
what other bootable systems are available, write a new grub
configuration, and possibly update the MBR and stage1 of Grub ...

Now somewhere there must be a configuration which hard drives' MBRs and
stage1's have to be written.

I think my machine is configured to write this to multiple drives. (I
vaguely remember configuring this long ago during an installation or
maybe an upgrade from debian to Devuan)
In any case it tries to write to /dev/sdb and has trouble doing so.
(hardware problem; I plan to remove this disk once I find a
screwdriver)
/dev/sda, /dev/sdc, and /dev/sdd are also available.

Where is this configuration located?
How can I change this configuration?

-- hendrik