The way we have it working right now is just to instantiate a TFile and write the TTree separate from the object. ROOT was crashing for me if I filled the TTree object from binary then ask it to write the TTree to a TFile using the TTree::Write() call. I believe the correct method of filling large (>1Gb) TTrees is to instantiate a TFile then the TTree such that the TDirectory the TTree is linked to is a real file. As you fill the tree it writes to file such that you don’t end up trying to write a multi-Gb TTree to a file.