I ordered and installed what I thought were (3) 3TB SATA drives in my HP Z800 yesterday, and they came up as 4TB drives. Then I checked them again and realized I was shipped 4TB drives when I ordered and paid for 3TB drives. Newegg.com substituted the 4TB drives probably because they were out of the 3TB drives. I was amazed they work, because everything I have read said 3TB is the absolute Z800 max. Now I'm curious if 6TB or 8TB drives will work on my Z800.
I'm using Win 7 Pro, 2x Xeon X5690 CPUs, 24 GB RAM, Z800 MOBO ver. 2, built in controller, 1100W PS, and 2x 3-tube heat sinks. Plus I'm using the Intel Rapid Storage Technology Utility and Driver for Microsoft Windows found at
https://support.hp.com/us-en/drivers/selfservice/hp-z800-workstation/3718645 in the Driver-Storage folder. The (3) 4TB drives are configured as GUID Partition Table (GPT).
Here are the Hitachi 4TB SATA drives Newegg sent me: https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=Hitachi+GST+Deskstar+4TB+&N=-1&isNodeId=1
This helps me a lot, and added 3TB for a total of 12TB of storage (not including C:\). If I need more storage I can add a 2-port
SATA controller and (2) more SATA drives plus USB drives.