Today’s Question: I know I need to plug in my external hard drive when I want to work in the Develop module, but is it OK to make changes in the Library module without connecting the drive?
Tim’s Quick Answer: Yes, you can most certainly apply metadata and other updates in the Library module, with the exception of the Quick Develop section. There’s no need to worry about applying updates in the Library module when the source photos aren’t available.
More Detail: All updates you apply in Lightroom Classic are saved to the catalog, so technically the source photos don’t have to be available to apply updates. You can view the images based on the previews that have been rendered, and then update metadata via the catalog.
By default the exception to this is when applying adjustments to an image in the Develop module or with the Quick Develop section in the Library module. To provide the most accurate preview possible, adjustments are based on the source image on the hard drive. That is why adjustments in the Develop module are disabled by default when you select a photo that is not currently available.
For the metadata updates in the Library module the source files don’t need to be available. You can update any metadata you’d like, and those updates will be preserved in the catalog. If you have enabled the option to automatically save metadata updates to XMP, then the metadata for the source files will be updated automatically whenever the source images are available to Lightroom Classic again.
Note, by the way, that with the XMP option enabled, metadata updates for proprietary raw captures will be saved to an XMP sidecar file, while updates for other supported image formats will be saved directly within the source image file.