preseed.cfg, remastered ISOs and checksum issues

["Adam Baxter" <debian@voltagex.org>]

Hi,
I'm sure I'm doing something wrong here but I'm seeing errors I've never seen before.

I'm rebuilding https://deb.debian.org/debian/dists/stable/main/installer-amd64/current/images/cdrom/mini.iso using something along the lines of

sed -i '1s/^/timeout=1\n/' boot/grub/grub.cfg
sed -i '1s/^/default=0\n/' boot/grub/grub.cfg
sed -i "s/--- quiet/auto file=\/cdrom\/preseed.cfg DEBCONF_DEBUG=5 language=en country=AU locale=en_AU.UTF-8 keymap=us/" boot/grub/grub.cfg

xorriso -indev mini.iso -outdev mini.mod.iso -map boot /boot -map preseed.cfg /preseed.cfg -boot_image any efi_path='/boot/grub/efi.img

So the boot parameters are:
vga=788 auto file=/cdrom/preseed.cfg DEBCONF_DEBUG=5 language=en country=AU locale=en_AU.UTF-8 keymap=us

This results in the following error:
The file needed for preconfiguration could not be retreived from file:///cdrom/preseed.cfg. The installation will proceed in non-automated mode

If I change it to file=preseed.cfg I get 
The  checksum of the file retrieved from <preseed.cfg> fails to match the expected value of "". The file may be corrupt, or the provided checksums may be out of date

Note that https://www.debian.org/releases/stable/amd64/apbs02.en.html says that the checksum is *optional*

The other issue I can see is that if I get into a shell, preseed.cfg is not at /, or /cdrom or /media

I'll try adding it to the initrd as a workaround but I thought remastering the ISO was supposed to be an option, too.

Thanks,
Adam