copying usb images or iso : error 254 in disk images

Hello,

I am repeatedly trying to copy usb to usb using the Restore function in disk utility (ie, just copying the whole disk image)

I am also trying to unpack an iso onto a usb

However, each time I try, i get 1 of 2 error messages:

"could not validate source - invalid argument"

and "could not validate source - error 254"

any ideas ? thankyou