mjhjmtu

I created a bootable USB drive with the Debian net installer for amd64. I would like to include some non-free firmware in the installer but the filesystem of the installer on the USB drive is read-only.

I know there is an installer that includes non-free firmware, but for learning purposes I would like to know how to make it writable so I can create my own customized installer.

First I tried the good old chmod:

$ sudo chmod 777 -R 'media/maxim/Debian 9.4.0 amd64 n'

But I got the following error for every file and folder on the USB drive:

chmod: changing permissions of 'Debian 9.4.0 amd64 n': Read-only file system

A bit of Googling learned me that chmod needs write access to change the file permissions so chmod won't work anyway for what I want to achieve.

Then I tried the following command:

$ sudo hdparm -r0 /dev/sdb1

/dev/sdb1:
 setting readonly to 0 (off)
 readonly = 0 (off)

But that didn't work, it's still read-only. I also tried to do it with /dev/sdb instead of /dev/sdb1, same outcome.

Then I tried to remount it with rw, but that also didn't work:

$ sudo mount -o remount,rw /dev/sdb1
mount: /media/maxim/Debian 9.4.0 amd64 n: cannot remount /dev/sdb1 read-write, is write-protected.

So basically my question is: how do I make the filesystem of the Debian installer written to a USB drive writable?

The installer is an ISO9660 (CD-ROM) filesystem. There's no way to make it writable. You can't modify it in place. You have to rebuild it.

At a low level, you want to copy the files into a directory on a writable filesystem (e.g. ~/myinstaller), make your changes, and create a new ISO image with genisoimage or xorriso. You'll probably have to add some options to make the image bootable.

If you don't want to get your hands that dirty, get the debian-cd package which is the thing that builds the official Debian installation images, and use that to build your custom installer. Or see if the customization you want can be handled by simple-cdd.

The statements by Wumpus are correct, except that the Debian installer
promises to be prepared for using a firmware add-on partition without
the need to re-pack the ISO.
See https://www.debian.org/releases/stretch/amd64/ch04s03.html.en ,
at the end of paragraph 4.3.1, about "sdX2" and "tar".

Said this, the xorriso options which were used to create a Debian netinst
ISO are published in a file of that ISO named "/.disk/mkisofs". Many of them
are for the Jigdo production and begin by "-jigdo". You may omit them.

If you want to get boot option proposals for some bootable ISO, run

xorriso -indev debian-9.4.0-amd64-netinst.iso -report_system_area as_mkisofs

Instead of the proposed "--interval:local_fs:...-netinst.iso" you may use
file path "/usr/lib/ISOLINUX/isohdpfx.bin", or cut yourself an MBR template
file from the ISO:

dd if=debian-9.4.0-amd64-netinst.iso bs=1 count=432 of=isohdpfx.bin