returning to 8.2 from 9.0

[Fbsd8]

Have test pc. Been running 8.2 just fine. Installed 9.0 from cd and 
after playing around tried to reinstall 8.2. Got error about GTP table 
while trying to setup the HD. Used 8.0 livecd to
"dd if=/del/zero of=/dev/ad0 count=5 to wipe the start of the HD clean.
Now 8.2 install issues this msg
GEOM: ad0: the primary GPT table is corrupt or invalid.

How do I fix the HD so 8.2 will install?