Reduced HDD Partition

My computer:
Intel Core 2 quad, 3GB RAM on ASUS P5P43TD Pro mobo
OS: XP pro SP2
System32\drivers\atapi.sys\properties version 5.1.2600.2180
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Servic es\Atapi\Parameters\
EnableBigLba\0x00000001(1)

I partitioned a Seagate 1.5 TB SATA harddrive into 100GB for
OS/applications and the rest for data, both as NTFS. AdminTool/Computer
Management\Storage\Disk Management shows two partitions of about 100GB
and 1300GB, both Healthy, as expected. Harddisk Manager (Paragon) also
shows the second partition being about 1400 GB and active, as expected.

However:
Window Explorer's Properties shows that the second partition is only
about 800GB and almost no free space remaining. (I have used that much.)
Why is Explorer indicating a reduced capacity? Where is the remaining
about 600GB hiding? How can I make the whole partition avaiable?
Appreciate help.