Start by identifying the device name.
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdc 8:0 1 14,9G 0 disk
├─sdc1 8:1 1 2,6G 0 part /media/myth/Ubuntu 20.04.1 LTS amd64
└─sdc2 8:2 1 3,9M 0 part
In this case it is
/dev/sdc. Proceed to unmount it:
sudo umount /dev/sdc1
Write your ISO file to the drive:
dd if=/home/myth/downloads/ubuntu-20.10-beta.iso of=/dev/sdc bs=1M oflag=dsync status=progress
When not using
oflag=dsync, it is wise to call
dd exits to ensure kernel buffers are flushed and all data is written to the device. The kernel might do some caching and buffering to do I/O optimization and if you unplug your drive prior to the last blocks being written you might have a bad time.
About the flags
bs=1M replaces the default block size of dd to significantly increase throughput. Going above 4096 does not yield significantly better performance normally, but when using the
oflag=dsync setting instead of let's say
conv=fdatasync, it has a much greater impact, as data is periodically synced instead of just before dd exits.
status=progress is of course to display the current write speed, elapsed time, and how much data has been written.
oflag=dsync does a write + flush after each segment, which makes
progress display the actual throughput.
conv=fdatasync ensures a flush when dd is done, right before it exists, meaning
progress might show write figures greatly skewed by kernel buffering.