iTunes: How to move [or copy] your music [library] to a new computer [or another drive] - http://support.apple.com/kb/HT4527 - a somewhat bewildering and not always easily understandable set of options.
Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Open iTunes and immediately hold down the Option (alt) key (shift on Windows), then guide it to the new location of the library.itl file in the moved iTunes folder.
Windows users see tip at: https://discussions.apple.com/message/18879381
iTunes for Mac: Moving your iTunes Media folder - http://support.apple.com/kb/HT1449
iTunes for Windows: Moving your iTunes Media folder - http://support.apple.com/kb/ht1364
another poster recommended do not move your whole library. Moving the media (as it says) simply relocates just the media files and not all the other files that are a critical part of your library. iTunes is more than just media files.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html