Depending on the version of FreeBSD, you may have a mix of gpt/mbr boot sectors. There are a number of utilities to zero out the boot sectors or it can be done manually with
dd(1). Be aware that many hard drives backup gpt from the first 2 sectors to the last sectors of the drive.
Search for "zero out gpt" and look at
http://www.rodsbooks.com/gdisk/index.html