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LadyKate - if you want to replace an existing format file, it is better to drag it from wherever it is and drop on the book details panel (where the cover is). It will overwrite the existing format file, update the size in the database, and do any other housekeeping Kovid deems necessary. The file you dragged-dropped remains in place - even if it is on the same drive. Much safer and you won't be chastised for 'going under the covers/skirts/kilts etc'
BR