How did you model the shaft and disk? Did you use solid elements? If so, how many elements go across the diameter of the shaft and the thickness of the rotor? Please reply with an image. If you have only 1 or 2 elements, then you will have some inaccuracy in your results. You need 4 or more elements across these dimensions to improve the accuracy. Quadratic elements will be more accurate than linear elements for the same number across the thickness/diameter.
It would be better to model the shaft with beam elements and the rotor with shell elements, then you don't need to worry about the question I asked above. You can model the unbalanced mass as a point mass at the correct radius.
The BC is very simple, Fixed. You can't get that wrong if you use Beam elements on the shaft.