Playing around today I shot an entire roll of 36 exposures and at the 36th frame the motor drive tried to advance the film more instead of releasing. This left me stuck half-cocked and I had to go into a darkroom to pop the canister and relieve the film tension so I could rewind the film. I thought the md12 auto-slacked the film when you finished a roll? Am I not understanding the operation of the drive?
half cocked meaning I can turn the manual film advance about 3/4 of the way to stop and then the film, being at the end of the roll and taught across the film plane, will not advance anymore and the release will not allow you to rewind the camera at this state. When I finish the roll the motor just kind of goes BRRRRRrrrrrr and you can kind of hear how it is trying to get the film to advance more.
The "rrrr" sound is normal. What you may've done is just wound so far that now the rewind button is working against the film tension. Dealt with that on Sunday with my Nikomat FTn. Grabbed a pen and used that to depress the button. Once I did that, I was able to rewind the film. You'll need to remove the MD-12 to do this, of course.