Lightroom (LR) is a combination of Camera Raw and a Photo Database. That means two things; you have the same retouching/editing tools as Camera Raw, but you also have a program that keeps track of your photos. Because the program keeps track of your photos you have to move, rename, and delete photos using LR. Working on the photos outside of LR will cause LR to lose your photos and that is when you will see the exclamation point!
There is no more crucial time in LR than when you are importing photos. This is where almost 1/3 of digital photo students lose track of their photos. You must choose your external drive as your destination and you must take your most recent version of the Library home with you so that you can bring it back to class.
LR Basics for the Classroom:
Your external hard drive should look something like this:
When you import - Check your destination twice: