This is annoying, and I have seen it before - when other people run ETAC, but not when I have run it. Probably because I use a Mac (mostly).
The failure to read these files means that the '5percent' files will not be produced (read the 3dttest++ -help output), but that is all.
What seems to be happening is conflict when the multiple instances of 3dttest++ spawned by the master program try to write the files simultaneously. I'll look at the code again, and try to figure out some way around the problem.
