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When you insert the USB flash drive into the Linux PC it is immediately recognized as a block device, not not necessarily auto mounted even when auto mount is enabled. It will be auto mounted if there is a partition that has a compatible file system. If it is auto mounted you need to unmount it from the command line while not "ejecting" it. We want the kernel to see the block device, however, we do not want any partition mounted. The block device will show up in fdisk -l and if it does not, then you will not be able to write the ISO to the flash drive. If the flash drive auto mounts, then simply unmount it by a command umount /dev/sdcWith the block device recognized, and the partition(s) not mounted, I found this command to work best: sudo dd if=./linuxmint-19.2-cinnamon-64bit.iso of=/dev/sdc bs=4M status=progress oflag=sync