Hello
Regarding Debian is not a supported distro, you should do some manual work. The KB is good but there are some differences between distros (e.g. Ubuntu uses mkinitramfs, not mkinitrd; I'd expect the same for Debian). Check the exact Debian syntax about how to install GRUB and rebuild the initrd image.
Another step that the KB omits is patching /etc/fstab file - the volume mapping usually differs from the source machine.
Here is similar discussion: Re: Conversion fails with the error:FAILED: An error occurred during the conversion:'root is not found'
HTH
Plamen