It shows that there is 527 GB of "unallocated space" on that hard disk (the shaded area). That's where your "missing space" is.
You need to right-click on that shaded area and choose "Create new simple volume".
When the volume has been created, right-click on it again and choose "Format".
When formatting is complete you will have a new drive-letter in Windows Explorer for that new volume. You can store files on it or install applications on it, just like you do on the C: drive.