Comment 3 for bug 1808421

From the description it appears that at one time the whole disk was formatted as zfs and that those file system signatures remain.

You can check what is going on with the following two commands:

  sudo blkid /dev/sda
  sudo wipefs --no-act /dev/sda

To fix you will likely need to remove the extraneous file system signatures. The wipefs command can help you remove the specific incorrect signature.

For reference, see:

zfs partition wrongly shows occupying the whole disk
https://gitlab.gnome.org/GNOME/gparted/issues/14