Well you are not following exactly the procedure I was talking about in my last message…you are calling Fill on the _b_grid branch of the first tree – I am suggesting that a cleaner way would be to create a third, output tree, that clones the original tree and in addition it adds the new ones (like here and here).
ssh to an lxplus7 node (newer OS than lxplus) and follow the instructions here:
ssh jasot@lxplus7.cern.ch
. /cvmfs/sft.cern.ch/lcg/app/releases/ROOT/6.10.02/x86_64-centos7-gcc48-opt/root/bin/thisroot.sh