I tried it actually that way:> Since you have the same disk layouts, I would rsync the data from the old to the new disk. Then update /etc/fstab with the UUIDs of the new disk. After that you're good to go.
- same HDD layout
- copy data (cp -a)
- insert new disk
- try to start
But that resulted in a non-starting system. It seems to me, that 'update-initramfs -u' is required.
>>- how to start the Debian installer?
> With a serial console, use tftp: http://www.cyrius.com/debian/kirkwood/qnap/ts-119/uboot.html
Any other way directly from the command line? Would be nice to 'activate' the installer running with the old HDD, replace the HDD (prepared as above) and then follow the procedure from http://www.cyrius.com/journal/debian/installer-flash-kernel.