Originally Posted by
he1957
By disengaging the motors from the rack/pinion you remove the mechanical contact so you might be more able to identify unusual noise sources. The video shows the left side is "stuck". With the motors free to turn with no load it should be easy to see if after power on both Y motors turn as they should, you can also check the bearings/ways for Y are not binding.
If after power on, the LH motor does not turn normally, swapping the motor plugs (after power off) at the driver ends should move the problem to the other side (be mindful of motor direction). If LH motor is still not moving it's the motor. Otherwise it's driver.
Alternately, you know the RH motor wants to move. Disconnect LH motor and plug RH motor into LH driver. If that doesn't move it's the driver module. Otherwise its the LH motor so repeat this with only the LH motor and chances are it won't move using either driver.
Since the LH motor seems locked its less likely to be a power supply issue to the LH motor but if the above doesn't isolate the issue then look there. (Don't have a wiring diagram to refer to for your machine).
This assumes, the controllers step/dir/pulse signal for Y is common for both motor drivers for the 2 Y motors.
Do NOT disconnect/connect motor driver plugs with power on!!!
Put plugs back to normal.
(That flexing of the gantry can't be good in the "broken" state :-))