Originally Posted by GeekyGal
Unboggling, FWIW, my approach to the long title / path names is to put the "extra bits" of title in a custom Subtitle / Subseries Column simply to get them out of the way. I sync my Calibre libraries between Win at the office and Mac at home with Dropbox.
FWIW, I found that Mac is more tolerant of long file and path names, but that "keep it short" works best. I had had a long persistent issue with the Library Maintenance > Check Library > Match to Database generating the same "extra title" errors over and over on three books. It finally dawned that all of the books involved had very long titles. Hand shortening titles in the Mac install then re-saving cured this. That never "stuck" in the Win Install, so the errors re-populated themselves.
GeekyGal, that part of your post didn't sink through my thick skull earlier. But it did today. I hadn't run Library Check for a few months. It caught about 20 invalid titles. I checked them. None were long titles. There were no visible problems with any of those books, no extra visible files. But most of them seemed familiar because I'd looked at those folders with keyboard shortcut "O" lately. So I tested some more on other books, and sure enough, just opening the author folder of a book makes it show up as invalid title, listed as author name in the list in Check Library. Deleting marked didn't delete any of the book formats, covers, or .opf's.
So I'd guess that OS X is doing something with invisible files and calibre is detecting them. I'm currently on Mountain Lion, and don't recall this particular thing happening with Snow Leopard, though I might have missed it in the past.
Anyway, so far my long titles are okay.
My book files don't go back and forth between different OSs. And they don't reside on DropBox or another server farm either. Seems I've got a plague of invisible files, and you had a plague of cross-filesystem incompatibilities.
Edit: Regarding long titles in filenames on OS X, whoops -- I was wrong. GeekyGal was right. I recently noticed trouble with long filenames when saving books to disk. So shorter filenames are safer.