As best as I can tell the on the drive that failed, it was the drive controller board that exists on every hdd that failed.
The drive would spin up, and initially the data cached to the ssd portion tried to load windows, but the hdd portion would not be recognized. Bios would throw a fit and freeze up for log periods on the main screen that showed the time, date, and drives in the system, but would act normal on the other screens, the drive also did not show up as being present. You could here the motor spin the platters up to speed. But the as far as I could tell the read/write heads were remaining parked after initial power up of the drive. No evidence of anything fried on the controller board, but then everything was so small you would need really powerful magnitudes to see if there was.
I have other drives fail before, things like head failure, Or the platters getting stuck, or not able to spin, or the drive just wore out and there increasing numbers of bad sectors even after a low level format. I have also had a drive where the main chip on the controller board started smoking and a small hole appear in the top of the chip when the drive was first powered up .but never had a previously good hard drive suddenly have the controller board suddenly fail, while I was using the pc.