cg0: bad magic number

[Dick Hoogendijk]

  My 1Tb sata2 drives work very well with FreeBSD. They can be sliced, 
partitioned and newfs can be run.
However, no matter what I try (sysinstall, gpart) when I slice my 500Gb 
drive and create a partition in it (1 block smaller than the slice) I 
can not newfs the /dev/ad8s1a
It always end up with a *cg0: bad magic number*

Googling I find a lot of references, but none give me the answer to my 
problem. All 500Gb drives can be formatted under Windows, linux 
(Ubuntu), Partion Magic, Solaris without errors. Alas, non of these 
systems supports formatting in UFS2. I need FreeBSD to do that but this 
fails... I'm getting quite depressed about this. Espescially because I 
cannot find the answer to this issue and I hate that! What on earth can 
it be?