It looks like the motor controller is trying to generate a Vio supply at coil #3. Maybe it senses Vio and refrains from generating the two Vcore supplies at coils #1 and #2 until Vio comes up to spec? This is the normal power sequence for most MCUs.

The -5V supply would appear at the anode of the diode near coil #4. You won't see anything there until the MCU starts to POST.

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The efficiency of buck converters can be very high, often over 90%, making them useful for tasks such as converting a computer's main supply voltage, which is usually 12 V, down to lower voltages needed by USB, DRAM and the CPU, which are usually 5, 3.3 or 1.8 V.

So I suggest tracing the 12V as well as the 5V and this will help you understand the logic board design better.

Keep in mind as well as buck converters usually run at 100 kHz up to maybe 1 MHz or so. A oscilloscope can find the signal controlling the switching. It could be hard disks can run the buck in continuous mode if the current drops to zero.

The buck converter is widely used in laptops which is seen with MOSFET switches, coils and lots of capacitors.

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