You guys are missing the fact that whenever you add a book to calibre, calibre creates a copy and adds it to your library, leaving the "original" where it was, so if the conversion does not go well you start over and add the "original" back to your library. for example I import a "*.mobi" that I convert to "*.epub" I want that "*.mobi" to be removed from my Calibre library as part of the conversion. The original "*.mobi" file that was imported to my Calibre library remains in it's original state, it is just removed my Calibre library.
The other thing being missed is I am asking this to be made an "option" if you don't want your original deleted then don't put a check in the box.
Maybe "Delete Source File" was a bad choice for wording, maybe it should say "remove original format from Calibre library".
If we don't want to add it as part of the program maybe a "plug-in" could be written to add this option.
I totally appreciate the hard work that has gone into this program.