Speaking of 4-pin Molex to 15-pin SATA cables catching fire, I thought this was a super interesting failure case so I did a bit of research into why they so commonly fail.
Thread.
Thread.
Just to be super clear about it, I'm talking about these things. They take a 4-pin Molex power connector from your PSU and turn it into a 15-pin SATA power connector for a modern disk drive. 
Back in the early-to-mid 2010s they started to gain a reputation for self-immolation.
Here's some delicious tech gore.
Here's some delicious tech gore.
People started making all sorts of wild speculations.
Maybe the current limit for the 4-pin Molex lines was greater than that of the 15-pin SATA lines? This wouldn't explain the actual failures, and also doesn't make any sense since they're all on shared rails.
Maybe the current limit for the 4-pin Molex lines was greater than that of the 15-pin SATA lines? This wouldn't explain the actual failures, and also doesn't make any sense since they're all on shared rails.
It turns out that the primary reason is due to the way that the SATA side of the connector is manufactured. Look at the size of the Molex connector pins, and the size of the wires, Now look at the size of the SATA connector. Those are some chonky wires to squash into there.
If you strip away all of the plastic, the SATA power connector looks a bit like this:
These crimped fins are held in place and then the plastic casing is injection molded over the top. This process requires a fair bit of heat and pressure, and it can melt the insulation on the wires. Normally this is fine because they're now encased in solid plastic.
But injection molded plastics are... kinda crappy? Any heat from the cables and environment, combined with any mechanical strain, can cause the plastic to soften and allow the cables to shift inside the connector.
Because the process is so slow, what tends to happen is that eventually the two bits of wire *just* manage to touch, causing a resistive short. It's usually not enough to trip overcurrent protection, but it's more than enough to dump a ton of power into that short point.
This is more than enough to melt the surrounding plastic, including that of your drive's power connector, and often even enough to weld the copper wiring together. Nasty failure mode.
With a normal SATA power connector you usually have the wires inline in a ribbon, which provides some mechanical strain protection. This mechanical protection isn't there with many of the Molex converter cables.
The converter cable types where the cable goes right through the connector, with blades in the connector crimped through the insulation, are much less likely to fail in this manner. It's mainly the ones where the cables are crimped with fins before injection molding that fail.
