PC Won't Boot with an SSD/HDD connected - Suspected Overclocking Issue?

Hi

tl;dr: PC won't boot when an SSD/HDD is connected, I suspect it could be related to previous owner doing overclocking.

I bought a PC-parts-combo (pre-built: motherboard, memory, CPU) secondhand, and then I built the rest of the computer with parts I had at home.

It works fine without an SSD/HDD when booting from a USB-flashdrive, but when I connect a disk to it, it wont even boot, it just shuts down immediately after pressing the power button. When taking out the disk, it boots just fine from the USB-flashdrive again.

The previous owner used the PC-part-combo for testing various graphics cards.

I looked around in the BIOS and when i go to the "MB Intelligent Tweaker(M.I.T.)" section, i get a warning/error prompt stating "The system has experienced boot failures because of overclocking or changes of voltages. Last settings in this page may not coincide with current H/W states."

So I have a feeling that the previous owner may have overclocked or done some sorts of tweaking.

When looking in the motherboard manual, it states this at the end of a section regarding the (M.I.T): "(Inadequately altering the settings may result in system's
failure to boot. If this occurs, clear the CMOS values and reset the board to default values.)"

So I think maybe the solution is to restore to defaults and then save that, but I would appreciate opinions on this before I go ahead :-)

In the (M.I.T) section in the BIOS there are some options such as:
F5:Previous Values
F6:Fail-safe Defaults
F7:Optimized defaults

Note: I tested with two different new SATA cables

Specs:
MB: Gigabyte P55-USB3
CPU: i7 870
Memory: 4GB
GPU: MSI GeForce GTX 980 Ti 6GB
PSU: Corsair RM750x