Here are the steps:
- become superuser
- # mkdir /mnt/data
- # sysinstall
- Choose Configure -> Fdisk
- If there are existing partitions press D to delete them
- To create a slice press C. To use the entire disk press A
- To commit changes press W. A warning pops up, choose YES.
- When asked to install a boot manager choose STANDARD
- Leave Fdisk by pressing Q
- Choose Configure -> Label
- Create a new partition by press C. Choose FS.
- Type in the mountpoint, e.g. /mnt/data. Take note of the drive device node, e.g. /dev/ad4s1d
- Commit changes by pressing W
- Exist the sysinstall.
- # vi /etc/fstab accordingly, e.g. /dev/sda4s1d /mnt/data ufs rw 2 2 in my case
Take a look on section 18.3 "Adding disks" of the official FreeBSD Handbook for finner details.