Installing on PowerBook G4 - grub failure

Hi all,

I'm sorry I've got to chime in, but I'm currently lost trying to install
Debian on a resuscitated PowerBook G4 - which got a new disk during the
process (M.2 128 GB "disk" in M.2-to-44-pin-IDE adapter) as the old one
must have been broken and removed...

I've started from CD, the image advertises itself as

 11.0.0 "Sid" Unofficial powerpc NETINST 20211006-11:28

and the install process proceeds nicely - until grub needs to be installed.
The guided setup created /dev/sda2 for /boot/grub (but didn't mount it into
/target!), /dev/sda3 for /, etc.

I suppose I've got to use the second console to fill in the necessary
actions to get grub installed, right? How? (And am I using the right disk?)

Any hints are welcome; I used to keep documentation whereever possible but
in this case too much time has passed :(

Thanks,
 Steffen