You may want to check your machine user manual to see if the storage is limited to 6 tb only.  This limitation is vary and depending on your settings with AHCI or RAID.  Ensure the machine to have the latest BIOS firmware as well.
My current build - T5500, win10 64 pro, 48GB RAM, does not recognize correctly my HGST 8TB HDDs formatted to NTFS GPT. While it shows the correct size in BIOS and in Intel Rapid utility during startup, it sees only 1.3TB by Win10.  My almost identical previous build - T3500, Win10 64 pro 20GB RAM, had two 3TB HGST HDDs by Toshiba HDDs, which worked properly in Intel Rapid Storage Technology RAID1 array. I can't figure where the snag is. 
Precision T5500. Wrong HDD size in Win10