Rear Cassette okay?
I recently replaced a cracked free hub body. When inspecting my XT rear cassette I noticed a small hairline crack on the inside of the cassette where it slides onto the free hub body.. The splines where the cassette and free hub interface are intact, but the crack is located on the innermost part of the cassette (on the metal before the splines start) that fits next to my wheel. I assume this happened when I cracked my free hub. Is my rear cassette shot? Is it safe to ride or do I risk damaging my new free hub body?
It's a judgment call.
There are enough splines that loss of strength in one isn't a safety issue, but it might add to freehub body stress. If the cassette is fairly new, and otherwise in great shape I'd consider using it a while longer and pulling it off to check from time to time.
But if it's fairly worn and you'll be replacing it soon enough, why risk damaging the new freehub body. Consider replacing it now.
Another factor is how you ride and how much you torque the larger sprockets. If there's lots of hard climbing on embankments and such, it argues for not riding a cassette which might split open under load.
The key to solving any problem is to understand and address the underlying cause.