Unable to change ownership on USB device

[Gerard Seibert]

I have several USB devices, all formatted with FAT32 or exFat. I can
mount them on my FreeBSD 11 machine; however, I cannot change the file
ownership on the device. I have a rsync script for backing up the
"/var/mail/vmail" and it subdirectories that are created by dovecot. I
cannot use the script to back up to the USB drive since it cannot
change the file ownership.

I hope I am explaining this correctly. My question is how do I change
the ownership of a mounted USB drive formated with FAT32 or exFat?

-- 
Carmel