HI,
Yes it is true that it does not work the first time. If you repeat the TTree::Scan command it will work.
So, what I posted before did not work, but this works for me (in both master and 6.04)
TFile f("hsimple.root");
ntuple->Scan("px:py:pz");
.> a.log
ntuple->Scan("px:py:pz");
.>We will investigate why it does not work the first time. This seems to be a problem with TTree::Scan. In other cases it works fine
Lorenzo