migrate system disk

[Edward]

> I have an old HDD which should be replaced soon, actually that HDD stands as my 
> system disk, what is your suggesion, how should I migrate the FreeBSD 8.1 from 
> the old disk to the new one?

I've used to do this a lot for server hardware migration, moving from 1
server to another new server. This blog post recorded what I tried & did :
http://scratching.psybermonkey.net/2010/01/freebsd-backup-and-restore-freebsd.html

Check it out,
Edward.