In fact, now it works.
I just add two function (that could be inlined to save speed) :
const int SDchipSelectPin = 4;
const int ETHERNETchipSelectPin = 53;
digitalWrite(ETHERNETchipSelectPin, LOW); // select ethernet mode -> LOW selects, HIGH deselects module
digitalWrite(ETHERNETchipSelectPin, HIGH); // deselect ethernet mode-> LOW selects, HIGH deselects module
It allows to set correctly the pins before using one or the other harware : Ethernet or SD. Ités a little tricky to program but it works. (I'm no yet sure if 'delay(10)' lines are important.
Another thing to test : Is it possible to let an file 'open' during et switch from sd to ethernet, doing some stuff and back to the sd to continue to write datas without opening again the file.
Bye, and thanks to this forum that got a so lot of informations !