This issue developed on my son's vehicle. We had the heads off recently and decided to remove the starter motor for a look at the drive pinion and ring gear. To my amazement, the motor was seized in the block and would not budge no matter how much spray/time/ tapping/cussing I gave it in the hope of seeing some movement/separation, but it didn't happen. Well, I did eventually but the motor was beginning to break, leaving the rear part of the pinion bearing housing still seized in the block. I then used a 4mm drill to remove about a quarter on the aluminium housing from 3 to 6 o'clock portion of what was in there and it finally loosened enuff to get it all out. That was a first for me - and I've removed a few starter motors over the last 53 years. The new starter went in with a smear of grease on it.
I should've taken a few pix but i was too frustrated to think of it at the time.