[Sirtel]

Yes, there's a risk of data loss if you use Calibre with the same library on several different computers. I lost some books years ago with OneDrive in that way. After that I ditched OneDrive and moved to Dropbox, but even then I occasionally had conflicts in the database (and I never synced anything with Calibre running). I knew how to fix those and accepted the risk, but yes, it can and will happen. Be warned.

The best practice is to only use Calibre with a strictly local library on a single computer, and keep a back-up copy in the cloud. FreeFileSync can do incremental mirror (one-way) syncing between the master library and the cloud copy. Calibre is not designed for simultaneous access on several machines; it's best not to keep an active library in the cloud if you plan to access it from other devices.