another day, another board revision!
this one has minor electrical changes from the previous one, mainly adding control over the spindle motor *power level* from the microcontroller. that way they were able to lower the average power consumption.
https://github.com/schlae/HardDriveInfo/tree/main/st251/21020
this is the control board for the ST-225 hard drive. it's an earlier revision. so far it's not too dissimilar from the ST-251 but the stepper motor driver circuit is quite different.