In trying to solve this issue I've tried using 2 different builds. Sorry if this gets a little lengthy.
Build A (approx 3 years old):
AMD HD 6950 Video Card
Intel i5 3340 processor
ASRock Z68 Extreme3 Gen3 Motherboard
1 TB Samsung HDD
Corsair HX 750W PSU
Windows 7
Build B (approx 2 months old):
Nvidia GTX 980 ti reference video card
Intel i7 4790k processor
MSI Z97 Gaming 5 motherboard
128 GB Intel SSD
500 GB Crucial SSD
2x2 TB Seagate HDDs
EVGA 750W PSU
I've tried several operating systems when dealing with the problem on this build.
The Problem:
On Monday September 14 I began to have multiple issues with Build B involving the video card and the Seagate HDDs.
Stressing my video card at all would cause my monitors to turn black and stay black until I rest my computer's power. Occasionally the computer would just restart entirely instead of just the monitors turning black. As well, neither of my Seagate HDDs can be accessed on Build B. Attempting to access them with the file explorer causes the file explorer to freeze up or for an error message to pop up saying that the drive could not be accessed because of an I/O error (no error code given). Occasionally one of the drives will be detected by the device manager as "Multiple Card Reader USB Device" rather than "ST2000DM001-1ER164" like the other drive. For reference this all happened while running Windows 10.
To give an idea of how stressed the video card has to be to cause one of these crashes, opening World of Warcraft on maximum settings (something the card was able to handle with no issues up until now) causes a crash before the login screen appears. Opening World of Warcraft on minimum settings causes no issues at all.
What I've tried:
On Build B, I've attempted uninstalling and reinstalling video card drivers without effecting the issues. I've also tried multiple iterations of the drivers dating back to versions released in June with no change. Reinstalling Windows 10 had no effect, as did downgrading to a fresh install of Windows 7.
I've been using my old build, Build A, to test parts individually. I've tried moving the 6950 from Build A into Build B and it worked with no problems at all. I've moved the 980 ti from Build B into Build A and it worked fine. Moving the Seagate HDDs from Build B to Build A causes them to be read and written to with no issues. I've moved all the parts back into Build B and the same problems occurred.
The fact that these issues occurred in separate components at the same time and that they work individually with my old build makes me think it's a motherboard issue. I've examined all of the pieces of hardware however and found no signs of physical damage. Any help would be greatly appreciated.
Build A (approx 3 years old):
AMD HD 6950 Video Card
Intel i5 3340 processor
ASRock Z68 Extreme3 Gen3 Motherboard
1 TB Samsung HDD
Corsair HX 750W PSU
Windows 7
Build B (approx 2 months old):
Nvidia GTX 980 ti reference video card
Intel i7 4790k processor
MSI Z97 Gaming 5 motherboard
128 GB Intel SSD
500 GB Crucial SSD
2x2 TB Seagate HDDs
EVGA 750W PSU
I've tried several operating systems when dealing with the problem on this build.
The Problem:
On Monday September 14 I began to have multiple issues with Build B involving the video card and the Seagate HDDs.
Stressing my video card at all would cause my monitors to turn black and stay black until I rest my computer's power. Occasionally the computer would just restart entirely instead of just the monitors turning black. As well, neither of my Seagate HDDs can be accessed on Build B. Attempting to access them with the file explorer causes the file explorer to freeze up or for an error message to pop up saying that the drive could not be accessed because of an I/O error (no error code given). Occasionally one of the drives will be detected by the device manager as "Multiple Card Reader USB Device" rather than "ST2000DM001-1ER164" like the other drive. For reference this all happened while running Windows 10.
To give an idea of how stressed the video card has to be to cause one of these crashes, opening World of Warcraft on maximum settings (something the card was able to handle with no issues up until now) causes a crash before the login screen appears. Opening World of Warcraft on minimum settings causes no issues at all.
What I've tried:
On Build B, I've attempted uninstalling and reinstalling video card drivers without effecting the issues. I've also tried multiple iterations of the drivers dating back to versions released in June with no change. Reinstalling Windows 10 had no effect, as did downgrading to a fresh install of Windows 7.
I've been using my old build, Build A, to test parts individually. I've tried moving the 6950 from Build A into Build B and it worked with no problems at all. I've moved the 980 ti from Build B into Build A and it worked fine. Moving the Seagate HDDs from Build B to Build A causes them to be read and written to with no issues. I've moved all the parts back into Build B and the same problems occurred.
The fact that these issues occurred in separate components at the same time and that they work individually with my old build makes me think it's a motherboard issue. I've examined all of the pieces of hardware however and found no signs of physical damage. Any help would be greatly appreciated.
... how about these odds, you had a faulty motherboard and your pcie slot on your motherboard is faulty? 
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