Having problems with winows 7 install

[Mrcheese]

This custom desktop is giving me hell.
Here's the build-
Evga 600w psu
Evga geforce gtx 960 4gb FTW gaming gpu
Asrock z170 pro4s lga 1151
Samsung 850 evo 250gb ssd
Wd blue hdd
Intel i5 6600k
Gskill ripjaw vseries 16gb (2x8)
I cannot install winows 7 pro x64. The hdd & ssd are not recognized by Windows 7, but only by the bios. I searched the forums did hours of trouble shooting. Tried cmd way, going through bios and making sure the settings are right.
I successfully installed it on the ssd by using another pc. When I put the ssd back in my pc I couldn't install the drivers as non of the USB slots worked.
I can successfully put winows 10 on it and run it but I already have a key for winows 7.
Can this be a defective mobo problem?
I've tested the install with a donor computer and worked just fine on my ssd &hdd.

 

[jimmyEatWord]

try changing the SATA connectors , do something random if you have only one ssd and one hdd , other than that it must be the motherboard 's setting , the bios , go to the bios settings and check everything , try resetting to the default and then check your boot options see if in hdd boot priorities you got it right , maybe they are all disabled , that was the case with my mainboard

 

[BadActor]

A little more complicated to install Windows 7 on a Skylake build since there is no support for ECHI. Have you tried Asrock's guide?

http://www.asrock.com/microsite/win7install/