This came up related to how an optional front top carrying handle could not be removed in a Z420, and the solution, HERE. I need to schlep around a fair number of tricked out heavy HP workstations, of all vintages, and had an idea this might fit all of them. Turns out it does.....
This device has HP option part number A9A48AA and part number stamped on the metal frame of 660542-001. These are available currently via HP and other on-line HP sources, and sometimes via eBay under that part number. Cost is about $35.00; I got mine for $25.00 USD off eBay, shipping included. They are heavy duty, as you would expect from HP, and this is a standard sized unit that slides into any 5.25" optical drive bay. It also has space for mounting two 2.5" form factor drives as you'll see, and is meant to go in the top 5.25" bay for best ergonomics. There is room at the rear for cable connections to added drives, but it can get a bit tight up there. I don't use any locking side screws on my 5.25" devices.
Here are a few pictures of it in use in a xw6600. The USB3 front ports you see there in the floppy bay are fed from the HP Texas Instruments based USB3 "2x2" card that has 2 rear backplane USB3 ports, and a card header for adding 2 more out front. That card has part numbers 661320-001 and 663213-001 on it, and there is an older NEC/Renasus one you don't want. I have been very pleased with that card and the Akasa 3.5" front form factor adapter shown below. Drivers for that card are available via a search for Z620 drivers, because the TI chipset used on the ZX20 workstations is identical to that used on this card (which was originally developed to add USB3 to the ZX00 workstations, but works for the xw generation fine also). If you add that card make sure to plug it in to a PCIe generation 2 slot and power it properly before you load the drivers. For the xw6600 the lower PCIe x16 video slot is what I use. You can check your service manual to see which are PCIe2.... the xw6400 only has PCIe generation 1 slots, so that card will run at 1/2 of full USB3 speed instead. Here's pics, which you can see after a moderator releases them:
