I can’t reproduce the problem with ROOT master:
C:\Users\bellenot\build\release\bin>hadd C:\Users\bellenot\Temp\test_hadd.root C:\Users\bellenot\build\release\tutorials\hsimple-test.12345-10 C:\Users\bellenot\build\debug\tutorials\hsimple-test.12345-09
Warning in <TClassTable::Add>: class ROOT::Detail::TTypedIter<class TEnumConstant> already in TClassTable
hadd Target file: C:\Users\bellenot\Temp\test_hadd.root
hadd compression setting for all output: 1
hadd Source file 1: C:\Users\bellenot\build\release\tutorials\hsimple-test.12345-10
hadd Source file 2: C:\Users\bellenot\build\debug\tutorials\hsimple-test.12345-09
hadd Target path: C:\Users\bellenot\Temp\test_hadd.root:/
Can you try to open the file in a ROOT session?