Hi there, to add big images (more than 4GB) to Berryboot, you should use a USB drive with EXT4 partition format, on Windows you can use Mini Partition Tool to format the drive. To read it you can use EXT2FSD. If you have access to Linux (can be Raspbian through your own Raspberry Pi device), you can use a software called GParted to format the USB drive. Let me know if you can do that on your end.
Alexander G Answered question