Thanks for all replies. I should clarify that I still learning the protocols associated with this forum. This thread was started (apparently by someone else), on my behalf (also apparently with the best intentions!).
It was the result of a somewhat casual comment by me about my failure to achieve what others have with a USB bootable drive.
The simple fact is that I have far too many other options running and I am perfectly content to work with DVD’s for this purpose. To the extent that to spend more hours on (maybe) achieving a USB that will boot an operating system is, to put it simply, not worth the proverbial candle.
Is it fitting, in terms of forum acceptance etc., for me to close this thread? I am asking because I did not start it.
Don’t worry about it. A moderator did a sensible thing to fork a separate thread. Nobody is fussed by a thread that is a bit different from the usual discussions about schematics, PCBs and simulation. I think all that is worth writing has been written so this thread will just peter out and close of its own accord in 3 months.
It’s actually quite fascinating if one is interested in filesystems. What ISO/USB image makers have achieved is an ISO9660 filesystem that is also a valid disk (probably FAT) filesystem. There must be enough leeway in the two specifications to allow such a hybrid image to be created. I must look into it out of curiosity one evening.
I did not read the whole Wikipedia article, but it does not look much like FAT. It seems to be heavily optimized for read-only data
That you can extract and ISO with dd pretty much means that such an ISO is just a binary copy of everything that’s on the disk.
I think you can directly mount such an ISO into a Linux filesystem, but that’s also quite logical in the Linux philosophy. If the software to work with ISO9660 is already available (in the kernel somewhere?) then the origin of the data dos not matter much to Linux.
But it’s 10+ years that I did much with CD’s, and that was before my permanent switch to Linux.
I have vague memories that you needed special software under windoze to make images of a CD and mount them, and that makers of CD’s were trying to thwart those efforts. I think that that image extracting software also liked to write the images into some non-standard format, so you could only work with those images with software from the same company. It was just a horrible mess that I never liked and never paid much attention to.
It’s also why I like Linux. It’s (mostly?) free of such conflicting commercial interests that result in horrible software convolutions. A disk image is just a binary copy, and you mount it just as (or very similar to) the original thing was mounted.
Actually there is one key piece of info in that WIki article. The first 32768 bytes of an ISO9660 filesystem is resereved for other uses. So a hybrid CD/USB image could hold a MBR and a small bootloader there. Additionally the root directory of the ISO9660 can be placed anywhere by the use of a pointer. So the USB filesystem could contain a partition to cover all the normal ISO contents. Such a hybrid filesystem could be mounted as either ISO9660 or FAT. One Linux software that can create this seems to be mkisohybrid. Gory details can be found for example here:
Partition type 0xEF is EFI, and partition type 0x17 is Hidden.
And the examination as an ISO image:
$ isoinfo -i openSUSE-Leap-15.3-DVD-x86_64.iso -d
Setting input-charset to ‘UTF-8’ from locale.
CD-ROM is in ISO 9660 format
System id: LINUX
Volume id: openSUSE-Leap-15.3-DVD-x86_64160
Volume set id:
Publisher id: SUSE LINUX GmbH
Data preparer id: KIWI - http://opensuse.github.com/kiwi
Application id: openSUSE-Leap-15.3-DVD-x86_64-Build160.3-Media
Copyright File id:
Abstract File id:
Bibliographic File id:
Volume set size is: 1
Volume set sequence number is: 1
Logical block size is: 2048
Volume size is: 2287671
El Torito VD version 1 found, boot catalog is in sector 20
Joliet with UCS level 3 found.
SUSP signatures version 1 found
Rock Ridge signatures version 1 found
Rock Ridge id ‘RRIP_1991A’
Eltorito validation header:
Hid 1
Arch 0 (x86)
ID ‘SUSE LINUX GmbH’
Cksum 71 1E OK
Key 55 AA
Eltorito defaultboot header:
Bootid 88 (bootable)
Boot media 0 (No Emulation Boot)
Load segment 0
Sys type 0
Nsect 4
Bootoff 12F0 4848