Answer by zvolkov for How can I use DD to migrate data from an old drive to a new drive?

You asked how to do it with dd, but I had better success piping the output of dump into restore. Given the source ad1s1a and the target ad2s1a:

$ mount /dev/ad2s1a /target
$ cd /target
$ dump -0Lauf - /dev/ad1s1a  | restore -rf -

I tried this on FreeBSD, actually I found it on the FreeBSD Forum