I downloaded to my Mac Etcher in order to burn a bootable USB drive for Linux. I got the User Interface and got as far as selecting the ISO file, which I did, but the Etcher User Interface did not appear to accept it and so did not light up the “Drive” portion so that I could then select “Flash.”
I tried it both ways - a USB drive then without a USB drive, plugging it in while the Etcher app was up.
As I mentioned, it failed with one software .iso (Linux Mint) and succeeded with another .iso (Ubuntu). Odd.
Do I understand you properly, that the USB should be plugged in before launching Etcher? I used the most recent copy, downloading it that day: Version 1.4.4.
The most probable answer is that the .iso file was corrupted - not complete - when I attempted to flash it. I was having some firewall problems and didn’t notice the file size.