GEOM GPT table is corrupt. Recover?

[Dick Hoogendijk]

  I wanted to install ZFS on two 1Tb harddisks. I did a fdisk -I 
/dev/ad12 to begin with, but:

GEOM: ad12: the primary GPT table is corrupt or invalid
GEOM: ad12: using the secondary instead -- recovery strongly advised.

OK, I want to follow up on this advice, but HOW?

The "corruption" probably comes from the fact these disks were used 
fully as ZFS mirror under OpenSolaris with an EFI label.

What's the best way to restore these disks to be fully used under 
FreeBSD (w/ ZFS).