I am in the process of updating an existing Intel DZ77RE-75K based system to a Gigabyte Z170X-Gaming GT motherboard. The upgrade required a new CPU Intel I7-6700K and new memory Gskill DDR4. In addition I replaced the PSU with a Corsair RM750i fully modular unit. At the time that I shut down the Intel system all drives, etc. were working as expected.After some initial 'got-chas' that were self inflicted (aren't they all) the Windows 7 64bit OS launched and I thought I had it all done.On initial start I had access to only one SATA drive, a Samsung Pro 250Gb drive on the mobo SATA controller. Happily that was the boot drive, or was I able to access because it was recognized as the boot drive? I shut the machine down and carefully checked all the SATA power and data cable - restart and still only the single drive. I could not even see my DVD rewrite drives. No power indication on either one.When I removed the previous semi-modular Thermaltake PSU I had no choice except to replace the 24 and 8 pin ATX cables. The remaining power cables I left in place assuming that there would be no difference in the power delivery from the different cables. When I could not access the DVD drives I opted to replace the Thermaltake cables with the Corsair ones for the DVDs. After doing so - reconnecting and reenergizing the system I restarted. I had power to one of the drives - not both. Since they are both on the same SATA power cable I would assume either none or both, one but not the other left me puzzled.Using the operational DVD drive I installed the Gigabyte utilities from the provided CD. I opted not to install any of the applications at this time. That install required s restart which I did. After the restart I still the original SSD and the later DVD. In addition I was now able to access a USB drive I had connected on a back USB port. Using that I installed the Corsair Link application and checked parameters on system, especially checking voltages etc on the PSU. That all looked acceptable -I then replaced all the power cables save the one that was powering the opsys drive. Reconnect, energize, power on. W7 up and happy - no additional SATA drives or DVDs.I do not consider myself a computer wizard. Nor am I a newbie. The last 20 years of my working career I managed medium size data processing departments for both non-profit and for profit. This is somewhere around the 10th machine that I have either built from scratch, or upgraded along the way. Now I will admit that I have little to no familiarity with UEFI, that would not seem to be the issue here however. I am not able to see the missing drives in either BIOS or W7What am I missing?