If a Harddisk is pre-partitioned - C:Partition wont identify the free space in the MBR/RDB container as useable.
Are you referring to free space in the MBR, or in an RDB container? C:Partition is designed to identify space in the former (and also within an EBR/extended partition), but not in the latter. Instead, it always creates a new RDB container for the one or two partitions it's asked to create.
Identifying unused space in an RDB container would be a useful feature to add, but the logic to find free space is already quite complicated.