How can I copy files from an ext4 partition on my USB drive to a fat32 partition on the same drive?

I tried mounting the ext4 partition as root, but it didn't work. My phone is rooted, but as it seems no one has managed to do what I want to do. I couldn't find any solution, that worked for me. How can I mount the ext4 partition? What I have tried so far: 1. Manually mounting the partition. I tried running mount -t ext4 /dev/block/sda2 /mnt/media_rw/usbotg, but that doesn't work and outputs mount: '/dev/block/sda2'->'/mnt/media_rw/usbotg': Invalid argument. I was trying this, because of this answer - https://android.stackexchange.com/a/220020 . 2. I also tried using vold-posix, but for some reason Magisk fails to install the module and shows Unable to unpack boot image, even though it is able to install Magisk updates on the boot image.