Maybe another idea … check “open files” limits in:
ulimit -H -a
ulimit -S -a
Try to increase it (in a shell in which you then run “hadd”), e.g.:
ulimit -S -n 4096
You could also try: hadd -n 1 ...
And you could also try to increase the “stack size” limit (also in the shell in which you run “jobs” that produce partial files as it is possible that some “job” dies because of it), e.g.:
ulimit -S -s 32768