Hi kovidgoyal and PeterT,
Thank for your answers.
Indeed Calibre stops automatically, and it did it, after more than 30 minutes (I tried to send more that 700 books).
So I guess it started to send the selection, and then realized that there was not enough place,so it stopped (similarly as the windows copy/past process in the past).
At the end I think, it just send one single book.
I actually know how much space I have on the external device, but I don't know how much I have to send...
It would be a first step that Calibre warn the user that there is not enough space.
Now I am trying to "save as" the selection in one single file on my computer, so that, I will be able to evaluate the total size from windows.
It should be possible to do like that, but a little bit cumbersome.
Thank you
Sylvain