Connecting SATA HDD into new gaming pc issue

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In my newly custom built pc, I have had it running for about 2 weeks now and I have the SSD as my main drive with my OS on it and such, finally my Western Digital Black HDD came in the mail and I was excited about installing it. So I install it into my Enthoo Luxe case and everything seems fine, but then I powered it on after I plugged it in. the pc would not even power on at all. No lights, no fan spins, nothing. NADA. So I unplug the SATA data cable from the motherboard that went to my HDD and, I had my HDD connected to the same SATA power cable that I have my LED/USB ports going to, since the EVGA cable the psu came with has like this daisy chain of connectors. I thought, oh cool I could just use that instead of buying a new sata cable.

So anyway after unplugging my HDD everything turned on as normal. I have tried plugging into different SATA ports, nothing. I have checked connection, nothing. I am really pulling my hair out over this, maybe someone sees what I am missing here?
Am completely new to all of this by the way, so any help is really nice. Thank you!
ALSO! Thought this might help, my PSU is EVGA 850 PRO, motherboard is Maximus VIII HERO.

UPDATE**** Now it wont turn on at all, even with the HDD completely removed. HELP! 🙁 all connections are in I have checked everything, I am so confused. Maybe it is staying off as a precaution because it doesnt like the HDD? HELP!!!!!!!

 

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NEW UPDATE!! PC turns on now, so stupid I just reconnected the MOBO power cable into the mobo, or sort of figited with it as it seemed loose. Which is odd because I didnt bump it at all while messing with the HDD business. Anyway, Now that my pc is running, I am afraid to try to HDD again incase it will not work. I am very upset, because I am wanting to add this storage. It is important.

 

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UPDATE 3!!! Ok I tried the HDD again and it works. Let this be an idiot guide to always make sure your cables are fully connected before freaking out. I was freaking out, in a fetal position crying thinking my 3000 dollar rig shorted out. Anyway. Yeah. Ahem. Cough cough. good bye.