Cassandra loading data from another table

[Richard Xin]

I have a tableA with about a few ten millions record, and I have tableB with a 
few thousands record,TableA and TableB have exact same schema (except that 
tableB doesnt have TTL)
I want to load all data to tableB from tableA EXCEPT for those already on 
tableB (we don't want data on tableB to be overwritten)
What's the best to way accomplish this?  
Thanks,