I did a search in this forum for "[Errno 21]" and found this response by Kovid, which is exactly what KevinH stated.
Quote:
Originally Posted by kovidgoyal
Ibooks stores epub files as directories, not actual epub files. calibre cannot import those. A bit of googling will get you some scripts you can use to convert the directories back into actual epub files which you can them import into calibre.
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