I ordered my hub without a cassette and ordered the same cassette that i run on my bike to ensure that there wouldn’t be any issue with chain length. I did play with tension on the quick release skewer to get the indexing pretty close and a 1/4 turn on the barrel nut adjuster for the rear derailleur and ran through the gears a few times and everything is quiet and shifts as it should. It appears the spacer provided is 1 mm while the spacer on many 10 speed bikes are 1.8 mm.
Hi folks! I experienced this problem too - new Zwift hub (8 speed) turned up, attached my bike, lots of grinding noises, not very pleasant.
Ended up taking my bike for a gear service which included replacing the chain and cassette and it’s fixed the problem completely 
My chain wasn’t super old and worn, but (like others have said in this thread) the chain and cassette tend to wear down together so the problem isn’t as pronounced until you move onto a brand new cassette.
Hope this helps anyone experiencing this problem!
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Same issue here. Took my bike in to repair shop and it was running sweet on the road back on the trainer, same grinding only smallest rear cogs, or if very low cadence on the high ones. Contacted Zwift support. After many videos they agreed to replace the hub. Same issue.
Yes, I have transferred cassette from Roadwheel, same issue. Yes, I have indexed front and rear derailleurs to death, and whilst I’ve minimised the issue, it’s definitely still there.
Usable, but not what I’d expect.
Apparently this issue is not limited to Zwift, I’ve seen the same reports with respect to Kickr Core too.
FWIW, I find using ERG mode in the middle cogs offers a very smooth ride
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I got a used Zwift Hub for Christmas and my first two rides feel grindy as well. My chain is relatively new, with 0.5 wear on the Park Tool wear indicator, and the cassette on the hub is nearly a 1:1 to what’s on my bike. Unit seems in good condition overall but I don’t doubt it could use some maintenance of some sort. I see nothing online about maintaining the internals of the machine, even when searching “JetBlack Volt maintenance/repair.”
I posted in another thread. I had similar experience of rumbling/grinding in certain gears that vibrated through the bike and could be felt in the pedals.
I noticed the 9 speed cassette I was using had rivets on the largest cog that prevented the spacer sitting flush and meaning it was out by about 1mm
I used a dremel and ground down the rivets and there has been a noticeable improvement to smoothness.
You can also order spacers designed for cassettes with rivets.
