TOPIC:

1977 KZ750 Twin. My starter will make the typical electric motor wine like it’s turning, but the output gear does not turn the chain. I’ve disassembled the output end of the starter and everything appears to be ok. Will it make that sound even if it’s not actually turning over? Or is there something else that could be wrong?

Are you sure the chain is not moving? Usually, the starter clutch is the problem. The starter motor and chain crank like crazy, but the clutch fails to engage, so the crankshaft does not turn.
If the chain really is not moving when the motor is, then it could be that the splines on the output shaft or the small sprocket have been sheared off, or the reduction gears could be stripped.
Has this problem been a long time developing, or did it come on suddenly?

I tried it with the stator cover off. It moves slightly then stops but keeps whining. All the gears look fine, nothing stripped, came on suddenly after sitting for almost two years.

Welp, my guess would be that something has gone amiss with the reduction gears, which live inside the starter.

I took the starter apart, and ring gear that the planetary gears sit inside spins in the housing. I don’t see how they made it to stay still in the housing, though.