I don't know if this is useful for FreeBSD but I'd like to add that sometimes you need to check if the mounted device is in use before unmounting it this can be done with the lsof command.
Also, you can check if the usb is mounted from the logs or from /proc/mounts or /proc/devices (usb_device) or /proc/partitions or from "df -h".
You can unmount the device directly if you still remember its name:
umount /dev/sdb
"There's always another way to do it" -- William Johny Cash Shakaspeare while doing a 360 loop in space.