[RPGoldman]

Many of these responses seem to assume that my library has simply been moved.

I assure you that this is not the case. As I said, I used both Unix "find" and the Mac's Spotlight file search programs. The epub files are gone.

theducks seems not to believe that the files were deleted. I assure you that this was the case.

I find on return that my backup disk has a copy as of late August with all the books. Sometime after that all the books are gone, although the metadata files remain.

This might relate to the posting of theducks saying that metadata.db should remain. That is, in fact, the case. The "Calibre Library" directory remains. It contains the metadata.db and .json files, but all of the books have been removed.

Similarly, wrt PeterT's response, the books are gone; the metadata.db file remains. And to JSWolf, no, the files are gone completely.

So my case is the exact opposite of what DoctorOhh conjectures: for me the metadata remains (although it contains no entries), but the books have all been deleted.

I stress that the books were deleted completely, rather than moved to the Mac's Trash folder, or located elsewhere on the hard drive, indicating that they were not removed by, e.g., my inadvertently dragging and dropping, or accidentally hitting the Cmd-backspace key (although note that my doing that could not have removed only the books while allowing the metadata to remain; the only easy way to do that would have been to somehow select all and only the books, which would be difficult to do even intentionally).

Thanks for your interest.