I have a number of disks in my FreeBSD box which exist only as backups of data.  I prefer that they not be mounted all the time, but instead mounted on use.  They are 2x internal SATA disks and an external USB3 disk.   This blog post explains how they are set up to auto-mount.

Firstly, my /etc/fstab entry for these disks looks like this:

/dev/ada5p1     /mnt/backup1    ufs     rw,noauto,noexec,nosuid,late            0       0
/dev/ada6p1     /mnt/backup2    ufs     rw,noauto,noexec,nosuid,late            0       0
/dev/da0p1      /mnt/usb1       ufs     rw,noauto,noexec,nosuid,late            0       0

All three disks are ufs, and noauto.

Next, my /etc/amd.conf

[ global ]
restart_mounts =   yes
unmount_on_exit=  yes

and my amd.map

# $FreeBSD: releng/10.3/etc/amd.map 164015 2006-11-06 01:42:11Z obrien $
#
*               opts:=rw,grpid,resvport,vers=3,proto=tcp,nosuid,nodev
localhost            type:=auto;fs:=${map};pref:=${key}/
localhost/backup1      type:=program;fs:=/mnt/backup1;\
                        mount:="/sbin/mount mount /mnt/backup1";\
                        unmount:="/sbin/umount umount /mnt/backup1"
localhost/backup2      type:=program;fs:=/mnt/backup2;\
                        mount:="/sbin/mount mount /mnt/backup2";\
                        unmount:="/sbin/umount umount /mnt/backup2"
localhost/usb1      type:=program;fs:=/mnt/usb1;\
                        mount:="/sbin/mount mount /mnt/usb1";\
                        unmount:="/sbin/umount umount /mnt/usb1"

In /etc/rc.conf, we need to start amd, and the portmapper:

portmap_enable="YES"
portmap_flags="-h 127.0.0.1"
amd_enable="YES"
amd_flags="-a /.amd_mnt -l /var/log/amd /host /etc/amd.map"