@eschwartz : thanks for those precisions

ve just made a try with one author and it work well as it created a "A" folder a sufolders for the author but there's a difference from the usual author's folder because in my calibre library I have only one folder for this author with subfolders in it for each books but when I've exported it it created 3 subfolders for this author because she has written a few books and 2 of them are with 2 other authors so there is one with her name only and the 2 others are with her name & author "X" / her name & author "Y" ...
can you tell me howcome it went that way ?
Can I have only 1 folder for the author as it was before ?
thanks again