clearing a drive

[dave]

Hi,
    I've got a corrupted drive which i am unable now to clear. Whenever i
try to fdisk it then relabel it i get an error that partition c does not
cover the entire disk and that this might interfere with system utilities.
I've tried:
dd if=/dev/zero of=/dev/ad1 bs=512 count=1
and various combinations thereof, without success. Normally running
sysinstall will work, but in this case it is saying an error occurred
writing partition information to drive ad1, and not allowing me to proceed.
    This is on a 5.2.1 box.
Thanks.
Dave.