In DiskMgmt Snap-in on Windows 2003 the “% free” column displays an incorrect percentage when there is more than 20.48Tb of available disk space on the disk

This is a display bug in diskmgmt.msc console.  You may ignore it and use File Explorer to view the %Free Space.

This is fixed in Windows 2008/Vista and beyond.

As friendly reminder, we also recommend you upgrade the server to Windows Server 2008/R2 for supportability since Windows Server 2003 SP2 is no longer in Microsoft's Mainstream support.