The clutch outer basket is directly attached to the crankshaft. There is no way you would be able to turn the clutch basket by any means. Even if you took off the clutch cover and had all the plugs removed from the cylinders, you would barely be able to turn the clutch outer basket by hand. Certainly not by engaging 5th gear and turning the back wheel or the universal joint. It would simply be too hard to do. So do not draw any conclusions from that test you made.
On the GS1100G (Which has a different gearbox layout), it is the clutch inner hub that is attached to the front gearbox shaft, which then transmits power (through the appropriate ratio) to the rear gearbox shaft which then powers the bevel gear which turns the shaft drive. In the GS650G, the front gearbox shaft, or main shaft transmits power to the rear gearbox shaft, or lay shaft, and then back up to another section of the main shaft (not connected to the first section of the mainshaft) and to the bevel gear.
In this picture below of the GS1100G (which has the engine upside down) the inner clutch basket is attached where the guy's hand is holding the shaft (as the actress said to the bishop) - the front gearbox shaft.
In this picture you can see the bevel gear at the end of the rear gearbox shaft but the bevel and the secondary bevel has been pulled.
I think most us here would agree that it's highly unlikely that there is a break along either of these shafts or that the bevel gear has stripped. If you can select the gears (as you have indicated because you said you put it in 5th) then I would also suggest that none of the gears in the gear box are stripped. If they were, there would be all kinds of metal bits grinding around that would probably have seized your engine within minutes.
BTW the pics above show a GS1100G engine layout. The GS650G has a slightly different arrangement with it's bevel gear. It is internal to the engine compartment rather than living in it's own chamber, which you can see here. It also has the bevel drive on the front rather than rear shafts:
But the principals are the same. However is does have that spring assembly on the right hand side of the front shaft (number 6 above) which I'm guessing is the drive cushion.
So where is your disconnect? The first thing to say is that if you have ever had an engine out, or even just had the clutch off, you will notice that if you were to spin the front gearbox shaft by hand (labelled 1 in the pic above) and the gearbox is in gear (any gear) there can be almost a quarter of a turn. I have put together a video to try and show it. This is on a GSX1100 / GS1150 EF transmission but it gives you an idea :
The first time I came across this I thought there was a problem with my gearbox. It turns out it is normal. If you add further slack or play from the bevel drives (minimal) and the cushions in the rear drive (if there are any), you might find that you can turn the rear wheel more that a quarter of a turn before it stops (if it is in gear). That amount of slack or play is disconcerting but not unusual. So don't be put off by this.
The only intentional disconnect is the clutch hub, the inner hub slips and does not turn the outer hub (if you are turning the rear wheel whilst it is in geaer) or the other hub turns and the inner hub slips (if the engine is running and the rear wheel is not turning whilst it is in gear).
I have re-read this thread and not found anywhere where you say you have tried to diagnose this problem with the engine running (maybe missed it). What happens when the engine is running and you select a gear and release the clutch? I assume that when you say you "can't get power to the rear wheels" you do have a running engine?
Given all of that, where are the unintentional disconnects? Looking at the GS650 layout, I would suspect that spring loaded cushion device shown in the picture. What would happen if that large spring shattered? I would imagine it would spin with the type of noise you describe. There would also be bits of metal in the whole of the engine, if the broken parts were able to migrate there. I do not know the GS650 set up so I couldn't say. It looks like tom203 has one apart and could tell.
There is also the possibility I mentioned in a previous post that the splines on the secondary gear may have given up (rather than the usual final drive in the rear hub). That would be where 3 meets 4 in the pic below. Or nut 1 has come off and the shaft has snapped at that point.
I don't know if any of that helps. I was just going through it in my own mind to try and figure out what was going on.
I will be very interested to find out what it was. Hope it wasn't a false negative.
