I had this problem before on a Sony Vaio I bout about 10 years ago. The tech's that installed the OS put it on a 19gb partition (c:/) and named the secondary partition (d:/) "DATA". I finally just backed everything up, deleted both partitions and formatted into 1 partition. I think he's wondering why his computer is saying two different things.
You can merge partitions by deleting your "DATA" partition, and merging them in Disk Management.
If anyone else can shed light, it would probably be appreciated.