Hadd & ClassDef: can't read merged files (error in header)

Dear experts,

I need to hadd together some bunches of few hundreds ROOT files containing histograms. All those files were generated from LCG jobs. The jobs were executed at different locations on the LCG grid, but with the same version of the LHCb software.

For some bunches of files I did not experience any problem. I could successfully create and read the corresponding “hadded” file.

However, in some cases I get the following warning during the hadd process: hadd_warning.txt (14.9 KB)

This results in the same warning when I open the merged rootfile, and in

whenever I try to plot or visualize a histogram contained in the file (those are not displayed at all!).

Since I have to hadd together about 900 files each time, is there any way to:
[li] check each rootfile individually before adding it (and, in case it returns those warnings, skip it), or[/li]
[li] recover the hadded rootfile, or[/li]
[li] recover the individual input files, or[/li]
[li] avoid this situation in the future?[/li][/ul]

Many thanks!