Problem withj dd

[Richard Owlett <rowlett@cloud85.net>]

I've downloaded the netinst iso with intention of copying it to a flash 
drive. I've done it before without problem.

> root@debian-jan13:/home/richard# dd bs=64k if=/home/richard/Downloads/debian-9.3.0-amd64-netinst.iso of=/dev/sdc
> dd: failed to open '/dev/sdc': No medium found
> root@debian-jan13:/home/richard# ls /dev/sdc
> /dev/sdc
> root@debian-jan13:/home/richard# 

What's happening?
[or perhaps "What have I forgotten?"]
TIA