Most likely there was a hybrid-ISO image installed before and the ISO-9660/ECMA-119 Volume Descriptor is still detected.
To verify this assumption first detect your disk with parted -l, Most likely a /dev/sdX.
If
is printing .CD001 at the beginning the Volume Descriptor is there.
Fat Warning: only do the following on an empty disk or with a recent backup!
Erase the Volume Descriptor with: