I assume you have only 1 disk available. If you have a RAID setup of multiple disks, see the reference at the bottom of the page.
Adding the Hard Disk to FreeNAS
In the Web GUI: Go to Disks -> Management
Click on the little + symbol near the bottom right of the table.
Select the correct HD from the drop-down menu. You should be able to recognise the HD from its size (i.e. larger than the USB stick).
Set the power-saving measures you want (disable all power-saving for Flash memory devices). Leave transfer mode as Auto.
I would recommend you leave it unformatted (and format within FreeNAS to UFS).
Click Add near the bottom of the page when you are happy with settings.
Formatting the Hard Disk in FreeNAS
To format: Go to Disks -> Format
Select the correct disk (you will not be able to select the FreeNAS USB stick), and then click Format near the bottom. All the defaults are OK (use UFS file system, and 8 as minimum free space).
Remember this will delete all data on the drive selected.
Enable (mount) the Hard Disk in FreeNAS
Now enable the drive for use by FreeNAS by mounting it:
Go to Disks -> Mount Point
Click on the little + symbol near the bottom right of the table.
Select the newly formatted disk from the drop-down menu.
Enter a Share Name so that you can find the drive contents on your network. e.g. FreeNAS1 (I would advise against using any spaces in any names if possible). This will be referred to as ShareName for future examples on this website.
The other defaults are OK (Type Disk, Partition EF1GPT, Filesystem UFS, Read-only disabled, File system check disabled, Owner root, Group wheel, Mode all ticked)
Click Add near the bottom of the page.
Now it's time to decide what you want your server to do.
Ref: Novell Developer