@mitu This worked! I was surprised, because after formatting the flash drive it looked like the drive had a single partition taking up the whole volume. I wasn't able to create partitions on Windows 10, so I used the Disk Utility on macOS to create two exFAT partitions (screenshots of the volume in Disk Management before and after below). After that I was able to follow the instructions to create the proper directories and transfer the ROMs!
Thanks!
Screenshot 2023-12-14 212355.png
Screenshot 2023-12-14 212425.png
Screenshot 2023-12-14 214455.png