I had issues trying to write the retropie image to the sd card a couple of times after there was already a retropie/linux installation on the card. I had to delete both partitions manually, then recreate and format to fat32 again because win32 imager wouldn't overwrite the card, it was wrongly making me choose one of the two existing partitions to write to.
Also, unless I'm mistaken, it sounds like you merely want to copy roms to the Pi itself, not run it off of a usb stick permanently, so just follow these 8 steps : https://github.com/RetroPie/RetroPie-Setup/wiki/Transferring-Roms#usb-stick