Check with HP for BIOS updates for your system. I have not heard of this particular issue, but I recall other PCIe slot issues that have been resolved by BIOS updates. Let us know if a BIOS update fixed the issue for you.
I have exactly the same Problem and I can confirm that it is still not solved in the latest BIOS version (01.35 Rev. A, 21 Feb 2012). It definitely is a Problem of the HP z210 Workstation because in a different Computer the two cards work just perfect. I already tried to disable the Boot Agent but this piece of s... engineering art seems to ignore it (again this works perfectly on the other Computer).
I was wondering if the thread creator found a solution to this or if there is anything else I can try?
Oh well, here comes the solution. It seems to be a BIOS Configuration Problem.
Go into BIOS by pressing F10 on startup (don't start with the two cards inserted or else you will not even be able to get in the BIOS), navigate to the "Advanced" Menu, select "Slot Settings". A blue Window pops up where you can enable or disable "Slot X Option ROM Download". Disable this Option for the Slots you want to put in your two cards. I have no Idea what this Option does but now my Workstation is booting and the cards are working.
Please keep in mind that I'm running the above mentioned BIOS version and I have disabled the Intel Boot Agent on both cards with the Bootutils (I don't know whether this makes any difference or not).
Hope this helps others with the same problem.
Thank you for posting this solution. This makes sense, because the option ROMs take up system resources. A lack of system resources can cause a failure to boot.