Split it and replace bearings before fitting it
The boxes are pretty reliable,but occasionally you get thrown a curve ball like this one,the replacement box for the above bike will have a rebuild prior to fitting,
Failures get expensive when other parts suffer collateral damage
You don't get any warning signs on the impending failure with the box,the bearings being sealed don't contaminate the oil as much ,this bike was serviced 4K miles ago and the oil was clean.the failing bearing meanwhile runs in it own little grinding pot.
The input bearing collapsed on the A1 30 plus miles from the wkshop,the only thing keeping things in line was the clutch push rod, for a while anyway
Getting my gearbox off the engine was a nightmare. The upper left dowel tube was solid and (unlike the bottom right dowel), there is no way to wedge the cases apart.
I eventually tapped out the gearbox bolt hole (M10 or M12 can't remember) then turned the threads off the end of a long bolt and used it to jack the gearbox off the engine. Turned off bolt bottoms out in the engine side and the threaded part literally jacks the components apart.