About deauthorizing specific computers - https://discussions.apple.com/thread/3809988 - Yes, if you have physical access to the computer.
iTunes: How to move [or copy] your music to a new computer [or another drive] - http://support.apple.com/kb/HT4527
Quick answer if you let iTunes manage your music: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Start iTunes with the option (shift on Windows) key held down and guide it to the new location of the library.
Macworld - How to transfer iTunes libraries between PC and Mac - http://www.macworld.com/article/46248/2005/08/shiftitunes.html
Move an iTunes library from a Windows PC to a Mac - http://www.macworld.com/article/1146958/move_itunes_windows_mac.html