If you are still using the EasyHax setup try the following.
Make sure you are using the updated version that runs from the RetroPie menu
Format the drive to NTFS and create a “roms” folder at the root of the drive. (do not transfer ROMs yet)
Run the Expand to External Drive (Disable) to remove any previous setups.
Plug External Hard drive to Raspbery Pie
Run the Expand to External Drive (Enable) - Pi will shutdown.
Plug External Hard Drive into your PC. There should now be a full selection of folders in your “roms” directory. Place your roms into the desired system and it should be good to go.