Jim, I use Carbon Copy Cloner, a terrific Mac OS X back-up utility to updates the files on my backup drives. The first time you clone your main music drive to another one it will take quite a long time to do it if it’s a large drive. However, from then on, every time that you clone it it just checks what files changed in the main drive and updates only those files on the backup drives. This helps me keep perfectly mirrored both my iTunes and Serato libraries. If you use Windows, there must be something similar over that side.
Regarding where the tags go, (e.g., genre, flip macro instructions, etc.) Serato saves them to the song’s file itself.
Tons could be said about the other questions you ask. Next-track selection skills evolve with time, not only practicing at home, but also performing in front of people. Throwing house parties is a great way to get going with that. Don’t delay playing in front of people!
Finally, I strongly suggest that you consider buying Digital DJ Tips’ <Master Class>. It will be the best investment you can make to get a fuller and deeper sense than the simpler How to DJ Fast. I speak as someone who not only took the course, but also goes back to watch again specific lessons from time to time.