known issue on our beloved 600s.
Some 600 gearboxes seem to live fine forever, but relative to other bikes ive worked on, 5th and 4th tend to fail more often. problem is well documented in this and other xt forums.
hurts, but I would change also 4th, of course you should do both pinions as a pair, not just the one where you can see the damage.
check also your oil pump as the steel debris might have passed through it
try not to lug the engine after you put it back together and add a magnetic sump plug for safety....
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