I bought a used Z800 with one harddrive for the OS. The boot drive is installed in an optical bay. It has 4 empty HDD slots to configure RAID
I added the drives, but not sure of the message in the BIOS for RAID that says all data will be deleted. I only want the drives in the RAID slots to be reformatted. I don't wantmy original OS drive to e overwritten. Am I worrying too much?

The HP Z 8xx series (800/820/840) have 3 separate mass storage controllers

2 Blue SATA 3 ports (uses normal windows driver built into the OS)
4 GREY "SCU" SATA ports (uses scu driver found in intel enterprise RST driver package for the "Cxx" series chipset)
7 WHITE SAS/SATA ports from onboard LSI controller chip (may need driver in pre win 10 OS)

the Z800 is based on the X58 chipset NO NVME SUPPORT IN BIOS
the z820 is based on the x79 chipset PARTIAL NVME SUPPORT - NO BOOT - DATA ONLY (bios 3.58 and higher)
the z840 is based on the x99 chipset (do not have this model so chipset may be wrong) FULL NVME SUPPORT

all Z 6xx/8xx systems can boot from a SATA 3 SSD (blue port) and the Z 8xx with the LSI chip can boot from SAS or SATA SSD's (white port)

most people using a standard windows 7/8/10 disk will be able to install the OS on the 2 blue SATA ports, and the 7 white LSI
without loading the Intel enterprise RST driver via "F6" during the OS Install

Please note the Intel RST enterprise drivers are different than the normal RST drivers in that they only support the Intel "C" series chipsets which are found in the HP Z 8xx series workstations
most of the above also applies to the HP Z6xx line of workstations

if setting up a system for the first time and you are using a stock disk without integrating the latest enterprise RST and LSI Drivers please place the boot drive and CD/DVD on the BLUE ports

if you integrate the RST/LSI drivers place the CD/DVD on a plue port and the boot drive on the grey SCU or white LSI ports

if not booting from the LSI ports, turn off this controller to speed booting, windows will still see and drives connected to it once the OS boots

creating a RAID array on the LSI OR the SCU ports will require that the controller "Initialize" the drives (IE- make the selected drives into a raid array) this step will destroy any data existing on the drives.

it will not touch your boot "C:" unless you selected this drive during the select drives to be raided step WHICH WOULD BE BAD IN THIS CASE!!

In other words for raid 5 you would need 4 drives in the system

1. your boot drive for booting the OS (do not select this drive)
2. at least 3 drives for the raid 5 selected in the create/select drives to be raided

if you want to boot from a raid array, have the drivers for the SCU/LSI chips on USB key/CD for loading during setup via "F6" or slipstreamed into the disk and create a array first, then proceed with the OS setup