Thx for your reply.
First I'm wondering how I start the debian-installer having the
unbootable 109 disk in the 209. Unfortunately I installed debian
already on the qnap 209.
I can boot that install with also the 109 drive in it, but how do I
launch the d-i then? And how do I make sure the commands you
suggested are executed for the right drive?
If you have Debian on the 209 you already have the correct ramdisk on
the 209 flash (both installs must be the same Debian version). Try
changing the UUIDs on the 109 disk to match those of the 209 disk and
see if it boots. If it does not boot it should drop to a shell where you
can try to find the error.