Problem:
$ sudo mount -t exfat /dev/disk2s2 /mnt/
mount_exfat: /dev/disk2s2 on /mnt: Resource busy
$ diskutil unmountdisk /dev/disk2s2
Unmount of disk2 failed: at least one volume could not be unmounted
Solution:
$ hdiutil detach /dev/disk2s2
"disk2" unmounted.
"disk2" ejected.