I have a USB OTG type C cable with a flash drive connected to it. The flash drive is formatted with FAT32. The phone can see it, TWRP can see it, but TWRP can't select it. In TWRP I try to mount it but when I press the option it doesn't get selected. It says "(OMB)" beside the option, but there's several GB of space available. I have an SD card that can be selected fine. I'm running TWRP 3.3.1 on LineageOS.
How can I chose to make a backup to the flash drive over USB OTG?