first of all i would spend the $12.00 on ebay for a ps2 keyboard, or buy a usb keyboard such as a logitech which will work
second i would download and read the z800 user and service manuals so i would have a basic understanding of what came with the z800 and how it's configured
third, why do people think trying to flash a bios when they have absolutly no idea what's going on is a way to resolve it
when it actually will/does cause even more problems,.....stop doing things blindly it will only cause you more problems
fourth, the z800 has multiple onboard controllers SATA and SAS
both controllers require the correct drivers , and knowing which controler is set as the main/boot drive along with the CD/DVD drive (if installed)
the most common setup is CD/DVD on SATA controller port 0 and boot drive on SATA port 1
however the drives can also be on the LSI SAS/SATA contoller ports 0 through 6 (see why reading the manual helps)
but the windows setup may not see the LSI controller, which is why the intel controller ports are recomended during setup. after win 10 is running you can move it over to the LSI ports
http://jp.ext.hp.com/lib/doc/manual/workstation/hp_workstation/504632_001.pdf
https://support.hp.com/us-en/product/hp-z800-workstation/3718645/manuals
https://support.hp.com/us-en/drivers/selfservice/hp-z800-workstation/3718645
windows 10 came out after the z800 chipset was released by intel, as such it will have all nessary drivers
the only driver that you might want to update/install is the Intel Rapid Storage Technology Utility and Driver for Microsoft Windows HP sp60733
if you get a no drives found on win 10 setup and it,s not on the LSI ports follow the directions below to load the nessary driver during the windows setup:
extract the files from the SP using winZip/WinRAR/7-Zip
locate the DOS folder, place it on your usb key, install the key on a usb 2.0 port and reboot/restart the windows 10 setup at the search for driver prompt, direct it into the usb key\dos folder, and it should now find the drive
while hp lists it as a win 7 driver, this is due to hp not testing the win 10 os on a Z800
intel who makes the RSTe drivers has tested it on XP-win-10
last, the Z820 uses the Intel® C602 chipset, ( similar to the x79 chipset)
and the z800 uses the Intel® 5520 chipset (similar to the x58 chipset)