DGroves,
For some reason HP changed their BIOS upgrade method with the Z440/Z640/Z840, with a new twist. They even put out their HowTo with new versions of BIOS for the prior ZX20 workstations including this new twist until I pointed out to them that it was only meant for the ZX40 generation and beyond (!). Here it is:
For the ZX40 and ZX G4 generations of workstations you can harvest the BIOS .bin file as you describe, but then you need to create a sequence of nesting folders into which you put the .bin file. For example, the latest Z440 BIOS is M60_0254.BIN, and you copy that into a folder you name New. Then you put New into a folder you name BIOS. Then you put that into a folder you name Hewlett-Packard, no more no less. That then goes into the thumb drive at its "top" level, and that goes into a USB2 port. These newer workstations actually also have native USB3 support built into their BIOS boot so you can use a USB3 port but I still prefer using a USB2 port so the habit is well established for me for older workstations that have both USB2 and 3 ports but no USB3 support built into their BIOS boot.
Why HP did that I have no idea, but it is necessary to use this nesting of folders.... the BIOS in these workstations is programmed to look for exactly that and if it does not see the nest it won't upgrade the BIOS. This same method will not work for the prior Z420/Z620 workstations because their BIOS is programmed to look for the .bin file right at the top level of the thumb drive.
We have so many different HP workstations that I made a thumb drive with one folder containing the proper .bin file or nest of folders for each type of workstation inside, and I just copy the correct thing from that folder to paste out onto the top level of that thumb drive.... that USB drive does not need to be blank of everything else for it to work. It just needs the right thing at the top level for BIOS to recognize.