"this BIOS not fully ACPI compliant, please contact your hardware vendor for a updated BIOS:

This is with CSM enabled and Storage OOPram policy: Legacy, sata mode ACHI

With your thought process in mind I tried a few things to no avail:

First the error occurs with original disk or patched usb   or patched ISO disk.

I've tried disconnecting both the DELL 2 button and wheel mouse, and Dell standard 108 key keyboard

I still get the error.

I tried disabling USB support in BIOS still get the error.

I tried ACHI mode or IDE still get the error.

If I put the machine in RAID mode then it will boot to the point where you select the drive but then it doesn't see ANY drives. Not the SSD, not the 2gig Sata II drive and not the Raid m.2. If I load the Intel RST driver it will see a drive but only the Raid drive and I can't use it no matter what I do. Can't create a system partition and clicking next, or pulling out the stick putting it back etc, nothing will get the operating system to install on the board.

I have installed the Win7 Patched USB version and I am at the desktop on the z170 OC Formula BIOS 1.7

I have to see if I can duplicate then post the exact BIOS settings I used to get it to install.

I did not setup a raid just enabled each stick and put sata controller in RAID mode but didnt' create an array. On disk 1 (m.2 stick 1) I was able to click the next button and it started the install. I installed Win7 without a hitch and I installed the lan drivers, the inf file and then service pack 1, netframework 4.5 and the IRST driver and the intel 530 video driver. I then went into the BIOS and told it UEFI only and to boot into that. It gave me the BCD error I had reported previously in another thread. I put CSM to enabled again and booted back no problem. I then tried to see if win7 would recognize a RAID array of the other 2 sticks. I selected them and created the array but there was no way to tell if I selected the correct 2 sticks or if I had selected one stick that was actually the OS I had just installed. Well, it wouldn't boot so I deleted the array but it also deleted the OS. So I probably did pick the wrong stick.

No big deal I thought. I tried everything I wanted to and now I needed to try and duplicate the win7 install process. So I began the install process again with all the same BIOS settings and I couldn't get the install to proceed past the point when you chose the drive to use. It kept reporting that it wasn't sure the BIOS could boot from this drive and that setup couldn't find a system partition or create one.

This "luck" has happened a few times. I could get the setup program to proceed occasionally and use a drive it said it couldn't. But 99% of the time I can't.

WHAT COULD BE CAUSING THIS?@?@?

Computers are usually pretty consistant. You give it that same info and it pushes out the same output. I can't trouble shoot a problem that seems to have a Random number generator attached to it. UGH>

I played around with the disk and the setup program a few times using diskpart. I cleaned them I created partitions. I converted to GPT and back I set them active and assigned a mount. I've even formated them. NOTHING seems to get the install to proceed.

Lost the keyboard and mouse at the chose your name screen but I rebooted 3 times and still no mouse so I went into the UEFI and changed 2 settings and got it back. Took some videos of the boot to desktop and now. I'm ready for bed. Been battling this thing for 2 weeks. I'm boxing in the correct BIOS settings to get Win7 installed on the z170s. 

I should try and duplicate it. but I'm so tired. time for bed

Well, the explanation for the ACPI non compliancy issue was, as usual, end user induced. I wish it was as cryptic as your explanation but it unfortunately comes down to "lack of attention to detail" and "RTFM" (read the fine manual). I was not paying attention to what sata cable I was grabbing as I was attaching devices and would occasionally attach it to one of the SATA ports that was "DISABLED" due to the m.2 card being installed on that shared lane. However, in my modest defense, the BIOS did show the drive as there as well as the motherboard monitoring UEFI tool. This would result in the blue screen ACPI warning.

Second thing, what did you mean by UEFI clear. Do you mean clear the CMOS?

I've installed Win7 about 7 times so far today trying different things. I've reached a problem though that I am trying to fix. I can't get the USB ports to work in Windows or it to recognize the BluRay drive. So I can't add any drivers or updates. For some reason the USB drivers are not being installed. I know they are supposed to be slipstreamed into the build by the patcher but they don't install. I wish I knew where they were or called so I could select it  after the installation completes I would just manually install the driver through the update driver process. I have a sneaky feeling that you have to have the USB ports active during the installation or windows won't install the drivers for them thinking you don't have that hardware in your system. So right now I'm trying all sorts of combinations of settings to see if I can get Windows 7 to install the drivers during the build process. No luck yet. Moue and keyboard work when I turn on PS2 emulation.