What will happen after adding another data disk

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Hi,
I know Cassandra can make use of multiple disks. My data disk is almost full 
and I want to add another 2TB disk. I don't know what will happen after the 
addition.
1. C* will write to both disks util the old disk is full?
2. And what will happen after the old one is full? Will C* stop writing to the 
old one and only writing to the new one with free space?

Thanks!