So if your USB device is /dev/sda, the sda1 is just a single partition on that drive. The boot loader is stored in the first blocks at the beginning of the USB drive: sda not sda1
The sda drive is typically the first drive used to boot an OS. So just a note, you probably shouldn't try to create an ISO image from an OS that is currently running. Files are changing all the time and could cause corruption. Rather, boot to some other OS to run the dd command (assuming it's a Linux OS you booted to). Try this, replacing sdX with the USB device path (do not use partition numbers at the end) you want to clone (use df -hT to list possibilities, but be sure which you choose!): sudo dd if=/dev/sdX of=ubuntu.iso This command copies the entire drive contents, the boot sector and any partitions. As an example, I have a bootable flash drive with Ventoy installed. If I want to make an .iso of that, I would first boot to an OS on my hard drive, in this case, Linux Mint. Next, I need to find the drive path: jbess@scooter:~$ df -hT df: /run/user/1000/doc: Operation not permitted Filesystem Type Size Used Avail Use% Mounted on udev devtmpfs 15G 0 15G 0% /dev tmpfs tmpfs 3.0G 1.6M 3.0G 1% /run /dev/mapper/lvmlmde-root ext4 428G 215G 191G 53% / tmpfs tmpfs 15G 98M 15G 1% /dev/shm tmpfs tmpfs 5.0M 4.0K 5.0M 1% /run/lock /dev/sda1 vfat 286M 5.8M 280M 3% /boot/efi tmpfs tmpfs 3.0G 176K 3.0G 1% /run/user/1000 /dev/sdb2 vfat 32M 28M 4.7M 86% /media/jbess/VTOYEFI /dev/sdb1 exfat 29G 23G 6.5G 78% /media/jbess/Ventoy In this case, */dev/sdb* is the drive path, and I'm ignoring the two numbered partitions listed there. Next, I would run the dd command: *sudo dd if=/dev/sdb of=ventoy.iso* Does that help? Another alternative is to use another bootable CD/DVD like Clonezilla or some other drive cloning software. Most can create an .iso from an existing file system. Jeremiah Bess On Thu, Feb 16, 2023 at 1:14 PM sami....@gmail.com <sami.st...@gmail.com> wrote: > I've tried the following, none worked... > > dd if=/dev/sda1 of=/ubuntu.iso bs=4K > mkisofs -o /dev/sda / > mkisofs -o /dev/sda1 / > > Those commands will create a the data o the iso, however the iso does not > boot. > > thanks, > On Thursday, February 16, 2023 at 10:54:47 AM UTC-5 Jeremiah Bess wrote: > >> Can you share the commands you are using leading up to the point where it >> doesn't work? >> >> Jeremiah Bess >> >> >> On Thu, Feb 16, 2023 at 8:53 AM sami....@gmail.com <sami....@gmail.com> >> wrote: >> >>> I've customized a Ubuntu 20.04 boot USB instance. The install works >>> fine and is completely automated. I can use dd to create a copy of the >>> USB, that is coy USB to USB. That works fine. >>> >>> However I need to create a file based image of the iso to distribute to >>> users. >>> >>> dd commands copies the data to another USB but does not make it bootable. >>> >>> An ideas? 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