Uh oh. I am a geek.. and I do spin the fans! Forgive me for I have sinned! (I also vacuum out my PCs).
Back to the thread at hand... this is good to know as I have a tendency to just keep everything. I also find that when I import a new book into calibre (that I've bought from a source other than Amazon), calibre stores it in the format that I purchased it. And when I email it to my K3, calibre converts it to mobi and stores that too. So I end up with 3 copies of the file: the original, a clean copy in original format, and the mobi version.
I could delete all but the mobi version, but they take up so little space (in the scheme of things), that I haven't bothered to do so. Yet.