You cant format a disk thats loaded and in use, or were you using FDISK on a different physical disc? Boot n Nuke is a very handy tool, great for wiping discs clean
It was an external drive.
Fdisk doesn't wipe the MBR. DBAN does. If the PGPMBR is still in place, then you will have difficulty using the drive. Another method is to use DD like so:
dd bs=4096 if=/dev/zero of=/Volumes/VOLUMELABELHERE