Most servos and steppers I've worked with are able to spin manually when no power is applied, but I've come across a couple which are stuck in their position. I'm not sure if they are just very difficult to spin manually, or if they would break if forced.
For example, I have this stepper and servo which seem impossible to turn manually.
Is there a specification that would tell you whether or not the motor can spin without power?
IMHO it depends on two factors:
if there is gear on the servo (so to motor rotates fast, the result is slow precise motion with high force), then moving it manually even little means to rotate the motor fast, which accumulats resistance araound the all way, multiplied by the gear ratio
if there is some electronics, then it can switch to "braking" state when powered off for security reasons, so moving it is much harder, than free rotating unconnected motor.
On many servos the two combines.
