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If I move my LR Catalog from the external drive, to my iMac's hard drive, will it speed things up? And are there any risks in doing so? What can I expect to have to do additionally to get it back up and running?
Close Lightroom and move the entire catalog folder to a location on an internal drive (it doesn't have to be your system drive). Double-click on the LRCAT file and Lightroom will reopen with your catalog as its default.
Alternatively, after moving the catalog folder, press CTRL while clicking on the LR desktop icon. A dialog box will appear, Press the "Choose a different catalog" button and navigate to the new location of your catalog. Once the catalog is selected, Lightroom will open.