OldBobD has made a great suggestion for you. Those tutorials can be a big help in understanding how LR handles files. People forget that LR utilizes both a catalogue file and a seperate file for your actual pictures. Those files can be almost anywhere, internal drive, external drive etc. LR just needs to know where they are and that they are associated with each other.

For beginners, having the LR Catalogue and Photo files on an internal drive is the easiest way to get started, assuming your hard drive is large enough to hold all your pictures. Many people have a second, large, internal hard drive just for photo storage. A second way is to have the catalogue on the internal drive and the photos stored on an external drive. This is what you started out with. You have also found the down side of this system. If you eject or remove the external hard drive and then reconnnect it, the drive letter may change. If that happens, the LR catalogue doesn't know where the photos went. You should see question marks on the photos and in the index on the left hand side. If you click on the top level folder, you should get a message telling you that LR can't find the photos and a button to "find" them. You can then browse and find that external hard drive and chose it and everything should be back to normal. Which USB port you use is irrelevant as drive letters are assigned in sequence, not by which port you use. The suggestions from posters who asked if you had plugged anything in before plugging in the external hard drive were spot on. If you want to use an external hard drive on a regular basis, the best way to handle it is to move your LR catalogue to the external drive. That way the two of them will always be together and the most you will have to do is to click on that hard drive from the LR library index on the left hand side. Many people use an external Hard drive because they run lightroom on BOTH a desk top AND a lap top in the field. This avoids any problems with unlinked catalogues.

I hope this helps to explain what happened to you and how to fix it. One of the most important things about getting started in LR is determining how you are going to store and organize your photos. Organization is one of LR's most important features. I strongly urge you to begin assigning "Keyword Tags" to every photo describing the who, what, where, why, when etc. That way, years from now you will be able to find every photo of "Aunt Jane" you ever took by simple doing a keyword search. Last but not least, set up a backup system and use it on a regular basis. Hard drives all eventually fail.

Good Luck

She needs to know exactly how to do this. I'd like to know exactly how to do it too...