Immediately make sure you have a good backup of your calibre library! You may be close to loosing your books and/or metadata.
I can see three possible causes, someone more familiar with this error message may be able to narrow it down:
1. The Dropbox software is active and has the file metadata.db open, perhaps in order to sync it with the other computer. Make sure that you never use calibre on more than one computer at a time, and that Dropbox is allowed to fully sync with the cloud before you turn the computer off or try to start calibre on another computer. Calibre is NOT intended to be used with networked filesystems or Dropbox. You have plenty of room for mistakes that can cause data loss if you ignore this.
2. The filesystem on the disk is corrupt. Disk maintenance software in your operating system might be able to fix it. But maybe not without data loss. If calibre works ok from the other computer, make sure to copy that library over after the filesystem has been checked and fixed.
3. The disk itself has failed. Disk maintenance software might be able to temporarily partially fix this, as a reformat also might be. But most likely you will have to get a replacement disk soon.
Last edited by Adoby; 07-11-2013 at 02:08 PM.