The Linux iso files (like Ubuntu) in my USB drive which is burned with dd could not be bootable, select “check disc for defects” from boot menu, finally it says one file is error, or failed to load ldlinux.c32…f…I have retry many times, only the very small iso files like openSUSE-Leap-15.4-NET-x86_64-Build243.2-Media.iso is correctly verified by “dd if= count= | sha256sum”, its strange. Sometimes it creates a file name as my device name /dev/sdb so I have to remove it. If using DD in guest OS in VirtualBox, then burning iso into USB drive is OK, so my USB drive has no problem. Is it caused by I don’t do it in the recommanded way ( && sync at the end of commandline) or how to fix this issue of DD.
Maybe you should start with showing the complete command you use to dump an iso to your pen drive.
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Be sure the target is the device NOT a partition on the device!!!
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many Linus distros require the iso to be modified to boot from a USB. NOT openSUSE it is boot ready if you copy image to the device.
You can also use other tools, as imagewriter (GUI) or live-fat-stick (console):
https://en.opensuse.org/SDB:Live_USB_stick
I bet the drive is mounted on the host and get unmounted when passed to VirtualBox Guest.
I have to umount every USB thumb drive to get a good dd copy. If it is mounted it gets trashed.
sudo dd if=kubuntu.iso of=/dev/sdb bs=4M
sudo dd if=/dev/sdb bs=512 count=<iso size in byte/512> | sha256sum
It is mounted on VirtualBox, but guest OS burning perfectly.