On a FreeBSD system, you can use the umount -f command to forcefully unmount a disk partition and get rid of the "device busy" error.
To forcefully unmount a disk partition, use the umount -f command followed by the name of the partition you want to unmount. For example:
# umount -f /dev/ada0p1
This will forcefully unmount the partition /dev/ada0p1.
Note that using the -f option may cause data loss, as it will forcibly terminate any processes that are using the partition. It is recommended to use this option only as a last resort, when other methods of unmounting the partition have failed.
For more information about using the umount command, you can refer to the umount man page by running man umount on the command line.