By checking the option of creation of a safeguard that causes to duplicate, before conversion, all the files to select. A folder can, in this case, being specified by traversing the folder . It is useful, by precaution, to check this box even if the treatment is slowed down by it.
On the other hand if you already made a safeguard before launching, you can uncheck this option.
Caution : if there exists a tree structure with under folders in the selected folders, this one is not reproduced in the folder of safeguard.