Your answer was very usefull and let me make a big step forwards...
Understanding the upload sequence is to me something precious...
I followed stictly your tips and retrieved a normal sketch upload with simple examples found in the basic directory.
I tried than to upload one my sketches freezing the MK module earlyer (playing with read / print Serial) and the problem appears again.
Here the error message during upload (that's still keeps uploading...) :
Le croquis utilise 27560 octets (10%) de l'espace de stockage de programmes. Le maximum est de 262144 octets.
Les variables globales utilisent 2884 octets (8%) de mémoire dynamique, ce qui laisse 29884 octets pour les variables locales. Le maximum est de 32768 octets.
processing.app.debug.RunnerException
at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:152)
at cc.arduino.UploaderUtils.upload(UploaderUtils.java:77)
at processing.app.SketchController.upload(SketchController.java:732)
at processing.app.SketchController.exportApplet(SketchController.java:703)
at processing.app.Editor$UploadHandler.run(Editor.java:2055)
at java.lang.Thread.run(Thread.java:748)
Caused by: processing.app.SerialException: Erreur de la commande « touch » sur le port série « COM10 ».
at processing.app.Serial.touchForCDCReset(Serial.java:107)
at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:136)
... 5 more
Caused by: jssc.SerialPortException: Port name - COM10; Method name - openPort(); Exception type - Port not found.
at jssc.SerialPort.openPort(SerialPort.java:167)
at processing.app.Serial.touchForCDCReset(Serial.java:101)
... 6 more