I would like to try ChromiumOS on my old laptop. Hexxah doesn't offer a CD image, only a USB one. USB sticks don't boot on this old thing and there is no reference to them in the BIOS setup. It's probably too old for there to be a newer version of the firmware that includes it, and even if there was it's too tricky to mess with just to try out an OS.

The answer would seem to be to convert the .img file to a .iso file, but I don't think this is as simple as it first appears. Is it possible? I wonder whether there is a method which wouldn't work for all USB formats to all CD formats but would work in the specific case of bootable images.

ccd2iso should do the job.

I have tried this but it wouldn't read the file. I have read some info on it and it seems to be intended for a particular sort of CD file not a USB img.

You could try loop mounting the chromium img file and then using the mkisofs or genisoimage commands.
In a terminal as root create a mount point: mkdir /mnt/chromium
Then loop mount: mount -o loop chromium.img /mnt/chromium
You would then run the mkisofs or genisoimage commands with options from your installed Linux system in a terminal in the /mnt directory.
I'm not familiar with Chromium so I don't know the bootloader it uses, isolinux?
The mkisofs commands will vary with bootloader and there are a large number of sites explaining this, probably posts here at LQ if you use the search function.

If you have questions, post back.

ccd2iso is for converting CloneCD images to ISO images, not USB images.
I would try to boot from USB using the Plop bootloader: http://www.plop.at/en/bootmanagers.html

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Thanks but this sounds way beyond my Linux knowledge and too much to learn for the purpose.
Thanks, this sounds worth a try, will have to get a larger USB memory stick.

If you have access to a newer system might try qemu with that image as hda.

The command qemu filename.img should work to boot to a vm.

This sounds useful whether or not it will run on that machine, thanks.