exactly. The problem only happens if in order:
1)I write on the txt file
2)I Leave the file written (without canceling an recreating it empty)
3)the sd is not phisically removed
4)The arduino reset
After all this things in the new setup() the card is not recognisez.
In all other case (I don't reset, I phisically remove the sd, I cancel the file before resetting, ecc...) everything works without problems