[vicwong48]

Thanks for the responses.

I have 24 GB ram and not displaying any kind of "pressure". (Eta: 3.4 i7, late 2012 iMac, plenty of drive space.)

I note the recommendation to not add files manually, but these are folders created from my prior use of Calibre; doesn't the "rebuild database" command use the opf files from these folders? (I could drop the epub files back into Calibre and let it rebuild the folder structure that way, but want to retain the metadata I've spent a lot of time on.)

I have a dozen or so plug-ins:
KFX Output
DeDRM
Source plugins: B&N, Fantastic Fiction, Fiction DB and Goodreads
Interface plugins: EpubMerge, EpubSplit, Goodreads Sync, Manage Series, Modify ePub, Open With, Quality Check, Resize Cover, Search the Internet.

I didn't push investigation of plugins since I had removed the preference files (including the plugins) in an earlier trouble shooting step when I reinstalled Calibre. That hadn't helped when it opened my 10k library (still spinning wheel). Also, the plugins don't seem to cause problems when I only have a hundred or so books in the library; there could be a threshold problem.

I'll try the debug. Will do some research on what to look for or test first.

I've always used the GUI, but got into the CLI because of this problem. I'm almost able to do everything I need to do from the command line (I do a lot of editing of the metadata), and have now gotten use to using the web client to view the database. The GUI is still preferable.

I don't know if there's a CLI version of the "download metadata" button, but what I've been doing in the meantime is opening a new blank library, adding my new books to it, clicking download metadata, book conversions and anything else that uses the GUI, then "copy to other library" to merge it with my old library. It's workable for now; I'm hoping a Calibre, OS or plugin update will magically fix the problem in the future (will also transfer to another Mac and see if it's still a problem.)