Hi all! Thanks for all the replies. Very helpful. I now have an updated ISO burnt to the flash drive now.
The challenge we have is flying blind and trying to work on the right key combination to get this machine to boot from the flash drive (without an Apple keyboard). We'll get there in the end. I hope! I also have to get hold of another USB serial port, but Amazon should help with that. I'll let you know how we get on. Thanks again for all the help. Paul. On Sun, May 31, 2026, 14:07 Thomas Schmitt <[email protected]> wrote: > Hi, > > Paul Duncan wrote: > > So, I would like to boot from a Debian live ISO with the following kernel > > parameters (set in grub.cfg). > > console=ttyUSB0,19200n8 > > > > Now, I can mount up the ISO in loop mode, and get into grub.cfg, but how > do > > I write this back to the ISO so that I can put in on a USB flash drive? > > After having copied /boot/grub/grub.cfg from the ISO to ./grub.cfg and > having edited it, you could create a new ISO from the original one: > > orig_iso=debian-live-13.1.0-amd64-standard.iso > new_iso=test.iso > > xorriso -indev "$orig_iso" -outdev "$new_iso" \ > -map ./grub.cfg /boot/grub/grub.cfg \ > -boot_image any replay \ > -compliance no_emul_toc -padding included > > See also > > https://wiki.debian.org/RepackBootableISO#In_xorriso_load_ISO_tree_and_write_modified_new_ISO > > (You are aware that "live-*-standard" offers no graphical desktop ?) > > ----------------------------------------------------------------- > For the more adventurous: > > Looking into /boot/grub/grub.cfg of > debian-live-13.1.0-amd64-standard.iso it should be possible to patch > the changed file into a copy of the Debian Live image. > > The following procedure is intended for a not mounted "$iso". > > Make a copy of the ISO as playground: > > iso=test.iso > cp debian-live-13.1.0-amd64-standard.iso "$iso" > > Copy grub.cfg out of the ISO: > > xorriso -indev "$iso" -osirrox on -extract /boot/grub/grub.cfg grub.cfg > > chmod u+w grub.cfg > > Learn the size of grub.cfg > > ls -l grub.cfg > > (mine says 1709 bytes) and memorize it, because your editing will have > to preserve exactly this size. > > Edit the copied file grub.cfg and insert the text pieces as needed. > Remove exactly the same number of characters from the remark lines > in the file in order to keep its size unchanged. Verify by ls -l. > > Learn the position of the file's content in the ISO: > > xorriso -indev "$iso" -find /boot/grub/grub.cfg -exec report_lba -- > > which will report on stdout something like > > Report layout: xt , Startlba , Blocks , Filesize , ISO image path > File data lba: 0 , 3285 , 1 , 1709 , > '/boot/grub/grub.cfg' > > Important is the number under "Startlba" (here: 3285). Multiply it by > the ISO block size of 2048 to get 6727680. > Now you can compose the dd run for putting your edited grub.cfg content > into the ISO: > > dd if=grub.cfg of="$iso" conv=notrunc bs=1 seek=6727680 count=1709 > > Mount the ISO and look into its /boot/grub/grub.cfg to verify that the > new content is present. > > ----------------------------------------------------------------- > > john doe wrote: > > I would start at > > https://wiki.debian.org/ManipulatingISOs#Remaster_an_Installation_Image > > This is about of the same age as the Mac, which would match. > But the shown runs of genisoimage and xorriso deal with the ISOLINUX > bootloader for Legacy BIOS, not with the GRUB bootloader for EFI. > > > Have a nice day :) > > Thomas > >
