[ubuntu] removing partition gpt files

I am trying to reload 14.04 using gmsl rescue but when I run the iso file listed in the Grub2 menu I get an error "installer needs to commit changes to partition table on the following mount points could not be unmounted /iso device "
I suspect that the media disk has gpt files on it because I mounted a Seagate exteal drive which uses the gpt file system.

I ran mount|grep ^'/dev' which listed the following

/dev/dsa2 on /type ext4 (rw, errors=remount-ro)
/dev/sda1 on /boot/efi type vfat (rw)
/dev/sdb1 on /media/michael/SEA_DISK type vfat (rw ........)

I have backed up my files.

I have looked at fixparts( to remove gpt files) and gparted( to remove the partition sdb1) but could someone confirm the best way with some commands as well.
Mike