Whereven on the T2 where you have the output from crab.
git clone https://github.com/Oviedo-PAF/nanoAOD-tools.git
cd xtools/rootfileReader/
Merge a single dataset into chunks of 2 Gb:
python haddtrees.py ../../CRABOUTPUT/SAMPLE/DATASET/ --outname NAME --maxSize 2000
Merge all the samples in a given production
paython haddtrees.py ../../CRABOUTPUT/ --prod myProd_DDMMYY --maxSize 1000