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Seagate drives are widely known for doing this, they are *very* sensitive to even the slightest power fluctuations. This "feature" causes the drive to immediately shut down when that happens, and it also happens a lot more often when multiple drives are connected to the same power supply.

The problem is somewhat compounded by your choice of ridiculously overspecced power supply. It's quite well known that Haswell CPUs make drastically fluctuating demands on the power supply and quite a lot of popular power supplies are too powerful and cannot regulate voltages accurately at the extremely low levels Haswell uses when idle. Given power supplies are overall worse performing and less efficient when used at extremely low load, it's not surprising that voltage regulation also takes a hit. You should probably look at replacing it with a Harwell-certified PSU, granted you don't necessarily need one but when you've combined it with extremely sensitive hard drives, it's the one thing I would look at first.

Given the current components in your machine a 400-450W power supply would be the most efficient, though 600-650w could be just about acceptable plus give you a bit of expandability for overclocking, etc. But even a 600w supply will be running at barely 5% load most of the time.