I found a tool to move your FreeBSD installation to a new hard disk named: clonehdd
Tool for copy partitions to another HDD. FreeBSD stores its data on partitions, which incapsulated in slices. Soft, such as Partition Magic, Acronis, etc. detect only slice, not real partitions on that slice. CloneHDD correctly understand size diference.
The port can be found in /usr/ports/sysutils/clonehdd
Example to move from ad4 to ad6
|clonehdd -src=ad4 -dst=ad6 -swap=2048 -fstab=ad6|