I have a 2005 manual R52 coming up on 200k miles. It's not a showroom car but I love it, and I would like to say that I keep good care of it. It's needed a clutch for a while and it started slipping pretty badly a few months ago, so I got everything I needed to do a clutch job and replace seals and I did it all in a weekend. It's by far the most difficult maintenance job I've ever done on a car, but it seemed to be successful. The clutch felt great, and I drove it another 2k miles without issue.

Until a couple days ago, when I was pulling out of the driveway to go to work, suddenly I just lost all power. There was no catastrophic noise, no weird engine behavior or warning lights, it was just like the clutch suddenly disengaged and wouldn't go back in. The throttle still worked and the engine sounded fine, but no power was making its way to my wheels. I could shift in and out of every gear without pressing the clutch pedal in. To summarize:
1) The engine is running fine and starts fine.
2) The clutch pedal feels exactly the same as before, no lighter.
3) Absolutely no power is making its way to the wheels.
4) There are no obviously catastrophic noises coming from the engine.

I ended up having to push the car back into the parking lot. Fortunately, I have another vehicle that I can use to commute in the meantime. Later, I put the mini up on jackstands and took off the front left wheel to get a good look at the trans. I'm not a master mechanic, but there wasn't anything that looked catastrophic even from under the car. A few things I noticed:
1) What looked like transmission fluid coated the underside of the differential.
2) The left halfshaft was fully in the diff.
3) With the engine running and in gear, the left halfshaft (I didn't get a good look at the right) was vibrating slightly, but not spinning.
4) With the engine off, turning the left rotor by hand DID NOT turn the right wheel. Turning the right wheel by hand DID turn the left rotor.

I had expected to find something obviously destroyed underneath the car but I didn't see anything besides the fluid. I thought I must have blown my diff somehow, but that seemed odd given the lack of destructive noises. I put the car back together and prepared to wheel it back in the garage when, to my surprise, it drove like nothing was ever wrong with it. I went to run a very short errand with it today, and I drive there fine, but it wouldn't move out of the parking lot when I went to drive back home. A couple other things I should note:
1) I say that I could easily shift into any gear without touching the clutch pedal. While this is true, putting it in reverse, 1, 2, and 3 is very easy while 4, 5, and 6 get progressively more difficult. I think I can hear the synchs struggling when I try to put them in those gears (while stopped). Those difficulties DO NOT EXIST when I do push the pedal in and shift.
2) While there are no catastrophic noises, I can hear a slight rhythmic grinding when I put it into gears 2 and up.
3) While replacing the clutch, I also replaced the rear main seal, the left input shaft seal, the left axle, shift cables, and shift bushings, all to OEM spec.

I would greatly appreciate help from anyone more knowledgeable about transmission issues than I. Why would it seem to completely break, work fine after just a little bit, and then break again? I would like it not to be so, but I fear I've messed up something in the differential.

 

Just now reading this for the first time; curious did you ever figure out what happened with / was the cause of the above issue?