post the output of 'blkid' (as root) to know current partitions alongwithIt looks like you have allocated the entire disk space as various
partitions. I guess by free space, you mean one of the partitions is
empty
or un formatted. Try deleting the partition which has the free space.
Make
it un allocated disk space. Then mint might recognize this.
this is my current partition - when I tried the dual boot, I had 40 GB
of free space also.