How to forcibly abort a command in a terminal window?

[Richard Owlett]

The problem occurred when using cp to copy contents of one 
partition to another.
I realized I had miss-specified options. As data was multiple GB 
and target was a USB connected flash drive I wished an immediate 
termination.

Cntl-C had no apparent result.
Closing terminal window apparently did not abort operation - as 
indicated by activity light on flash drive continuing to blink.
Unmounting device worked.
It seems there should be a better option?
TIA