I put in the HDDs with 2TB capacity each, Vista for OS, and connected to the RAID System with eSATA. Under RAID 0, my host computer detected total capacity much less than all the HDDs added up. Why is that?

Please check the host computer BIOS settings first. If the BIOS is set up as RAID+AHCI mode, both Vista and Windows 7 OS will detect the wrong capacity when the total capacity is over 2TB. Please re-set up the BIOS as Standard (Combined IDE Controller) mode or IDE compatible mode.