@speakintohe: I have around 4,500 books using 55GB; once one gets into heavily illustrated PDFs, for example, as many references and magazines are (even magazines often being well over 50MB) the overall size soon blows out.
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In my case I also have multiple libraries (for reasons I need not get into, but I can assure the Virtual Ones
that they are valid for me) with everything on the same drive. I have all the Calibre libraries as sub folders of the same top folder, all indexed for Windows Explorer searches which I limit to that folder and sub-folders so the search returns books only and is instant. One library is a temporary one.
That said, a Windows Library, as suggested by BetterRed, is a better suggestion for those happy with using Windows Libraries than my single folder with libraries in subfolders under it though and allows working across multiple connected drives (including networked ones) if that is wanted.
But if using Windows 8 the Search Charm>Files allows a readily accessible and effectively instant search through everything on even large drives so the libraries can be anywhere spread across it. It also automatically groups ebooks with other documents so avoiding the mess one gets with an Explorer search of a whole drive turning up irrelevant file types having the same search term in their names. This is what I tend to use now.