Legal assistant here. Regarding the sealed arrest record for Donaldson: I'm not in NY, but generally records are sealed for a variety of reasons. A lot of times your record is sealed if you had a favorable disposition. This could be that you were found not guilty, the prosecutor or police decided not to pursue the case, or it was dismissed for a variety of reasons. Sometimes the court will offer an adjournment of your trial for a period of time with the intent of dismissal. If, during that time, you meet all requirements the court has set forth (community service, rehab, etc), your case will be dismissed and record sealed after the set adjournment time ends.
Also, certain drug related charges will result in a sealed record if you complete a treatment program and whatever other conditions the judge sets.
There are of course other reasons that people have mentioned already. However, I don't necessarily find it odd that he has a sealed record, especially if he was a young student in NY when he was arrested.