I don't know about FreeBSD specifics,
But if you have a USB enclosure to put the drive in, you could probably write the the "memstick" image directly to the 500g drive using
dd(1).
The kernel/ramdisk should be loaded in memory, so you can safely "reinit" the drive once it's booted.
At that stage, you can grabs the sets from network/USB CD-ROM drive.