Disk Utility: Create Disk Image W/O Free Space

So far I can’t find a way to get the Gnome Disk utility to create a disk image without the free space: the two partitions I want to backup occupy at most 20G, while the whole disk is 4T. That means that an awful amount of time is spent backing up … nothing, really, not to mention that an even bigger amount of time is spent restoring free space.

Is it possible to do this with the Gnome Disk utility at this point in time? Note that I need to create a bootable ISO from my disk, in order to use w/Ventoy, which is why I need just one ISO file.

Alternatively, say I only backup the OS partition into an ISO file: how do I make that ISO file bootable, so I can use it w/Ventoy? Is there a way to do that without major headache (such as unpacking the ISO onto a USB, and making that USB bootable etc. etc. etc.)?

You can create a partition image using Create Partition Image rather than Create Disk Image. Use the cog menu in the bottom left of this screenshot, not the window menu:

But it’s not going to be bootable. That requires considerable extra work and I don’t know how to do it. It’s certainly possible, but not using GNOME Disks.

Thank you. I also tried using truncate --size to eliminate the free space from the big ISO file, as detailed here, but I ended up with a small ISO file containing only free space.

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