Knoppix 7.4 flash-knoppix command-line example

[fredvej]

@Porteño: /media/sdc1 is the mountpoint where you have mounted the file system in partition 1 on the sdc device. /media/sdc does not exist.

flash-knoppix wants a device to write on, not a file system. I guess that all partitions on the device must be unmounted to make the device available for writing.

Try this:
umount /media/sdc1
/usr/bin/flash-knoppix -m p -p 20000 /home/knoppix/Downloads/Torrents/KNOPPIX_V8.1-2017-09-05-EN/KNOPPIX_V8.1-2017-09-05-EN.iso /dev/sdc