can you see it in your fstab file? it might be being mounted by mintdisk.
if it is in the /etc/fstab file just insert a # at the beginning of the line that refers to the disk.
now reboot and it will not be mounted.
if mintdisk has mounted it and put an icon of it on your desktop, right click on it and choose unmount (or maybe eject).
now you can start gparted and delete the partition etc.