One of the highly desirable features of Lightroom is that editing is non-destructive. You can adjust, crop, retouch, and apply various settings to your images without loosing the original. However, it’s important to understand where the edit information is stored and the implications.

By default, Lightroom stores all metadata (including changes to the image) in its catalog. If you only use Lightroom on one computer and you never have any problems with your catalog, you’ll probably never notice – your changes just work. However, if you were to delete your catalog, all changes will be lost. In other words, you’ll only have your original image files. So even in simple use cases, it is critical that you back up your Lightroom catalog.

If you use more than one computer to work on your images, and use you a catalog on a portable hard drive, the same situation as above applies. However, since Lightroom will not use a catalog on a shared drive, the situation becomes more complex for those of us who share images across their network.

Lightroom, like many other photo packages, is also capable of reading and writing image metadata from image containers such as jpegs and from “sidecar” files. If you right-click on an image or folder in your library, select “Metadata” and then “Save Metadata to File”, the data will be written to either the appropriate place in the image file or an XMP file in the same directory as as the image. There is also a setting to do this automatically:  Edit > Catalog Settings > Metadata Tab and place a check in the box for “Automatically write changes into XMP”.  I strongly recommend that you turn this option on:

1. It increases compatibility with other applications including Photoshop.
2. If your catalog becomes corrupt, you can re-import images without loosing all your work.
3. Assuming you backup your images, you will also back up the metadata.
4. You can open images from another PC or import them into another Lightroom catalog and preserve all your edits.

If you’re starting to work with a second computer and haven’t turned this option on:

1. On your primary computer, right click on the top folder in your catalog and select “Save Metadata.”  It’s going to take a while, perhaps overnight, but it will write the XMP files.
2. Turn on “Automatically write changes into XMP.
3. You can now create a new catalog on your second computer and import your images into the catalog. Lightroom will pick up the XMP data.  Don’t forget to turn on the “Automatically write changes into XMP” option for each new catalog.

As you’re working, you might notice an indicator in the upper right corner of an image warning you that metadata has changed on the hard drive, presumably because you edited the image on a different computer or within a different catalog. You can right click on an image (or a selection of images) under “Metadata” and select “Read Metadata from File”.  If you’d like to force Lightroom to read Metadata at the folder level, simply click on the folder, choose Metadata from the main menu at the top of the screen, and select the appropriate option.

There are two drawbacks you should be aware of:

1. Writing XMP data to disk may reduce Lightroom performance in some circumstances.
2. If you have a lot of jpegs and write XMP data, each file will change (as opposed to raw files, where a separate .xmp file will be created).  If you use an online backup service, beware that writing XMP data en mass may result in all your files being uploaded again. On the other hand, your work will now be automatically backed up.

To recap:  By default Lightroom 3 stores all your work in the catalog only unless you configure it to automatically write XMP or manually force it to. XMP files are cheap insurance and for most users I recommend configuring Lightroom to write them automatically.