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Teeny - if you are using Windows 7/8/10
First you need to create a
Windows Library, which we'll name 'Ebooks' - you do that from Windows Explorer, so do a Start-Documents or Win+E to bring up a fresh WinExp window.
- in the Navigation panel (left) you should see Libraries, right click it, select New, type 'Ebooks';
- under Libraries you should now see something like Documents, Ebooks, Music, Pictures and Videos
- navigate to the folder (in the navigation panel) where you have all your books, right click it and select Include in Library->Ebooks;
- navigate back to Libraries->Ebooks and you'll see your folder structure in the centre (main) panel;
- right click in white space in the centre panel and select Arrange By->Type, you should see something like this (the blue folders are 'virtual folders);

- Now open the virtual folder for MOBI files, select all the files and drag them over to calibre and drop them into the book list.
BR