Shark: Data abort booting from disk

I keep getting a Data abort when trying to boot from the HD I installed in
a Shark.  I think this is from the old junk in the first sectors of the
disk.  Does this sound right?

I found an email in the archive which recommended the command:
dd if=/dev/zero of=/dev/rwd0a bs=8192 count=1

The problem is, when I try to do this I get a "read-only file system"
error.  /dev/rwd0a is rw by root, and this error occurs in: 1) single user
mode, 2) booting from the install kernel via the network, 3) after using
"sysctl -w kern.securelevel=0"  I've run out of ideas on how to zero out
this junk in the first sectors of the disk.  Can anyone help?

				Henry