Boot Camp Assistant asks that I remove USB

Hello,

I'm trying to flash Win 10 iso on a USB drive to use for a new pc that I built. I currently only have access to my Mac.

When opening up the Boot Camp Assistant, it requests that I remove the external drive - the specific message is '

External storage device Attached. Please remove any external storage device attached to this system before continuing.'

I have formatted the USB to MS-DOS (FAT) and 'Master Boot Record'.

Any advice on this please? All videos I've watched have allowed people to proceed.

Thanks