I have the retropie with fat32 partition to change the roms using a secondary partition….I make with this steps:
1-install fresh image of retropie
2-expand the partition and uodate retropie
3-using partiton wizard reduce the extended partition (ext4) of retropie (you need 4Gb of space for retropie and future updates) and format the new partition on fat32 (you cant access to this partition using windows, you need bootice (32or64bits) in bootice you click on parts manage and select thee third partition (maked by you) and press set accesible….now you can put your roms on your maked third partition….when you finish our manage of your roms go to bootice, parts manage, and set accesible the boot partition (first partition) of the sd.
4- You need mount the third partition on your etc/fstab file like other partition using winscp (wifi) or console
5-reboot and enjoy!!
((the process is easy when you have the partition mounted and maked because you only need set accesible the third partitin using bootice to manage your roms and restore boot partition to run retropie))
I try to explain the process to you using my english 😐 sorry for my bad english 😉