auto reset during the loop()

PaulS:

    for(i=0; i<TAILLE_NUM_UTI-1; i++) numString[i]=myFile.read();

Suppose you read past the end of the file. Then what happens?

I normally can not to be at the end of the file, I read the first 8 characters of a great line.

What if your file contains less than 8 bytes?

You really ought to be checking for unexpected results from read().