NT 4 Disk Manager and properties differ

I have a client with a Windows NT 4.0 sp6a Server running duplexed HDs (2 Adaptec 29160Ns and 2 Seagate 18Gb SCSI Wide HDs). When you enter Disk administrator, the disk shows a 4Gb C: Partition and 15.5Gb D: Partition, both are formatted NTFS… When you right-click on the D: partition, it only shows 8Gb total space. This is also the case in user space available… WHAT’s GOING ON??? How do I fix this and get their full space available???