Timely.... you can use 3TB drives just fine on the Z400, Z600, Z800 workstations.... I don't know about those 6TB WD drives.
If you want 3TB that is proven to work on the ZX00 generation I'd go to eBay and search out a SATA2 3TB drive like I did, or buy a HP brand SATA3 3TB drive that has the special HP firmware installed that lets the SATA3 drives work reliably in a SATA2 type of HP workstation (which the xw and ZX00 generations of workstations are). HP has clearly explained that not all non-HP SATA3 drives will work in their SATA2 generation workstations.
Take a look HERE.
And, below are some notes I made when I originally bought this drive new off eBay. As a SATA2 type of drive it should be compatible with your ZX00 workstations even if the drive did not come from HP originally. I personally always do the long DBAN low level of reformatting on a drive bought used off eBay, and I did that also for this drive because it came as a server drive with strangeness on its boot sectors that kept it from being identified by my workstation. The 3TB drives all will need a GPT partition to be seen as a single partition, and thereafter I always do a long type of NTFS format to map out any bad sectors. I have read that does not happen with a Quick type NTFS format.
All that will take you a few days, so be willing to take the time to do it right. I'd do this inside the workstation so you have proper cooling over this long run. I, also, have an external StarTech drive dock with a fan that blows over the drive in place externally (USB and eSATA interfaces). I'm probably being overly compulsive about DBAN but it has solved issues that nothing else has. Here are those old notes:
Same as: 614826-001, 628059-B21, 628180-001, 638516-002, MB3000EBKAB
It had HDS7230ALA AND H3U30006472S AND HUA723030ALA640 on its label, plus seemed to be a Hitachi Ultrastar 7K3000. It also had on its label HP 638516-002 and HP 397377-030
For use only as a data drive (not bootable due to BIOS limitations on the xw and ZX00 type of HP workstations) . Drives over 2TB need GPT partitioning table, not a MBR partition table.