The best thing to do is transfer the whole iTunes library to an external drive.
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.
You can, if you wish, just relocate media but this makes it tricky should you wish to relocate your whole library in the future.
iTunes for Mac: Moving your iTunes Media folder - http://support.apple.com/kb/HT1449