cla68:
I do not understand the reason why 4xSG90 plus 1xMG90 servos could be rotated manually but another MG90 not...maybe is a matter of tolerances among each gear or some other reason which I do not understand.
Friction is not well-behaved - there will be enough variation in the friction to tip the balance between
turnable and stuck, for a massed produced cheap reduction gear system. A very small amount of friction
is multiplied up by the gear ratio to a very large value if you attempt to back-drive.
If you need to be able to manually adjust when powered down, some sort of mechanical means should
be employed which doesn't involve rotating the servo shaft.