Internal, but I always run an SSD as an internal drive. It's much faster that way. I've got a 1TB SSD which is big enough to hold all of the music...
What are the specs of your computer? Anti-Virus software is a necessity but it can only do so much. Some software will actually slow your computer...
Glad I'm getting back into DJ'ing as well. Are you getting into live mixing now? Or will you be continuing to do CD's?
Basic Vol 2 - DJ Vypa / Ole Chamar - Always my go to!