Long time ago I posted on the forum about my work that allowed me to run a 3TB drive on a xw6600 on its SATA bus, which is not supposed to be possible. It took quite a bit of work, and use of a special Intel driver. The same storage controller ESB2 chip is in your xw8400 as in the xw6600, and the method would work for larger than 3TB drives. I concluded it was not worth the effort, and potential data risk, and chose to stick with 2TB drives if I really needed a large storage HDD in one of those ESB2-based workstations.
There is another way I'd go now, however. In the xw6600 I'd drop in one of the HP USB3 cards into its lower PCIe x16 slots and run a USB3 cable up to a spare OD bay where I'd have the large HDD mounted, with a SATA-to-USB3 bridge attached, fed by an internal power cable. It is pretty easy to find those, and a custom DC power feed would be easy to make rather than use a 120v AC to a DC power supply. I'd need to check the power capacity of the bridge and the large HDD max power draw. The 6600/8600 have PCIe Gen II for their 2 PCIe x16 slots; the 6400/8400 only have Gen I which still would run at 2.5Gb/s, in theory. That is up to 175MB/s based on HP docs for USB3 running via a 5Gb/s bus (300MB/s) which is the bandwidth of a PCIe Gen II slot.
If you can live with PCIe Gen 1 speeds then that bridge would work fine, and big drives via USB3 work great. I've gotten that USB3 card ("2x2") running fine under W7Pro64, W10Pro64, and now with the W10 Anniversary Edition. Very fast, and I can get you the HP parts numbers if needed.
When HP changed the forum around the old posts largely got lost, but I have a PDF copy I could send you if you PM me here, if you really want to try the SATA bus method instead. I'd advise against it.