Two disks that I had previously used on an mdadm mirrored raid were complaining about preexisting partition information and apparently thought they still belonged to a raid even though I had already zeroed out the superblocks. These disks had also been formated with GPT. Mdadm alerted me to the remnants when trying to setup a new raid.
GPT stores its protective MBR, primary GPT header and primary GPT table within the first couple of logical sectors on a disk. Additionally a backup secondary GPT table and secondary GPT header are stored on the last few logical sectors .