Can someone answer this about backing up files?

I just bought a My Book for Mac. I have not taken it out of the box yet until I find out if this is going to work for me. I have thousands of pictures on my Mac that I need to backup on the My Book. However, I would like to then delete almost all of the pictures that are on the computer while keeping the copies safe on the My Book. Can I continue regularly backing up without the pictures being deleted on the My Book? In other words, I want to use the My Book for a regularly scheduled backup task without losing any of the pictures that were backed up at an earlier date but are no longer on my computer. I appreciate anyone that may understand what I’m asking and provide me with an answer. I know I can use an external hard drive to store my pictures but I really want the My Book to regularly back up everything on the Mac. Like, is it possible to lock a file on the My Book so that it will not delete during backup because it is no longer on the Mac? (I’m just thinking about how iTunes works, where if you free up some space on your computer by removing music then you will also lose it off your iPod the next time you sync it).

You do not ever want to trust important data to just one drive internal or external. You need to have at least 2 copies to be safe. The forum is full of people ho did that and had a problem and lost everything.

Joe