HP seems to define what is "supported" as what they have very thoroughly checked and certified to work on their workstations. What is supported on one workstation may not be supported on another in the same family (such as Z800 versus Z600). That does not mean that other approaches will not work, it just means that only the things that have been tested to HP 's satisfaction are "supported".
I bring this up because in the most recent HP QuickSpecs for the Z600 I can find, v50, there is no mention of 3TB drives. In contrast, for the latest Z800 QuickSpecs I can find, v55, there is a listing of how 3TB drives can be used. This would be in the SATA2/early SATA3 era, but HP includes some SATA3 drives in the supported SATA drive listing (page 7). And, HP specifies that a 3TB drive in a Z800 needs to be attached to ports driven by the on-board LSI9260 Controller. See top of page 8 and the HP Technical and Service Manual, version 4.
But, things change with different BIOS and newer drivers, and newer hardware. HP will not be going back and certifying any of this.... we need to do it ourselves. The Z600 does not have a LSI controller on its motherboard, but I have my 3TB SATA2 HP drive working just fine on my Z600 (version 2). And, I have also posted about running the M.2 Predator PCIe-interfaced SSD that will work with one but not both of the currently approved HP Storage Controller drivers. You have to be willing to experiment.
The point is that we need to certify these things ourselves if we want to think and do out of the box. If I had a SATA3 WD Red 6TB drive to experiment with in a Z800 I'd attach it as a non-boot GPT partitioned NTFS formatted drive to "position 2" of the ports that are driven by the LSI9260 Controller.... refer back to that page 8.
It is somewhat hard to find the latest QuickSpecs for the Z600 and the Z800. These both will be for the version 2 motherboards. Here are versions 50 and 55, respectively:
http://www.bargainhardware.co.uk/content/specifications/z600_datasheet.pdf
http://www.bargainhardware.co.uk/content/specifications/z800_datasheet.pdf