Dear Paul,
I seem to be stuck due to the same issue with HP Z820 while attempting a fresh installation of Windows 7 x64. No Drives are found by W7 installation. Based on various posts on HP support, I tried a few dozens options, with no susccess.
In the step-by-step Windows7 installation guide on the HP support (https://support.hp.com/ie-en/document/c03465686), there's a mention to "Locate the boot controller" in the HP Z820 - in Drivers Storage section.
Out of 12 available drivers in the HP Z820 W7 x64 download zone, only three of them refer to LSI 2308 SAS Controllers (sp69010.exe, sp69177.exe, sp56513.exe).
Is there any possible way for you to assist me with this? which one is it more specifically
Many thanks
Eugen