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Also known as trashing the prefs, this is the act of forcing Lightroom to create a clean preference file in the hopes that it fixes whatever strange Lightroom problem you might be having. The preference file is where Lightroom stores all the settings you configure in the Lightroom > Preferences (Mac) or Edit > Preferences (Win) dialog. Along with all those settings Lightroom stores other data such as the location of your catalog file and which catalogs you recently opened among other things.

If Lightroom, or if your entire system, crashes shuts down unexpectedly this preference file can become corrupted, which can lead to problems ranging from more crashing to Lightroom not starting to all manner of odd and unexplainable behaviors. Sometimes a fresh do-over is just what the doctor ordered!

Here’s the basic steps:

1. Open Lightroom
2. Go to (Mac) Lightroom > Preferences or (Win) Edit > Preferences, and make a note of all your settings in your head or on paper. You’ll need to reconfigure those later.
3. Close Lightroom.
4. Navigate to the preference file (locations listed below) and delete it.
5. Restart Lightroom and reconfigure preferences.

When you restart Lightroom a fresh clean preference file will automatically be created taking you back to the default settings. Important: Lightroom stores the location of your Lightroom catalogs in the preference file, so when the new preference file is created Lightroom will no longer know where your catalog was located. It will simply look to the default location for catalogs and open the one it finds there or create a new empty catalog, which can be confusing. Make sure you know where your catalog is stored before you trash the preferences by going to (Mac) Lightroom > Catalog Settings or (Win) Edit > Catalog Settings and check the General tab to see its location. If the wrong catalog opens go to File > Open Catalog and navigate to the right one.

Here’s where you will find the preference file on different operating systems:

On Mac, open Finder and navigate to the [ username ] \Library\Preferences folder, drag the (LR1) com.adobe.Lightroom.plist and (LR2) com.adobe.Lightroom2.plist to the Trash. If you’ve installed Lightroom 2 Beta then also trash the com adobe.Lightroom2Beta.plist or the Lightroom 3 Beta, then toss the com.adobe.Lightroom3Beta.plist file too. If you don’t see the Lightroom plist file then you may be looking in the wrong folder (don’t go to Library/Preferences, go to [ username ] \Library\Preferences).

On Windows Vista and Windows 7, open Windows Explorer and navigate to the Users/[username]/AppData/Roaming/Adobe/Lightroom/Preferences folder, delete the (LR1) Lightroom Preferences.agprefs, (LR2) Lightroom 2 Preferences.agprefs, (LR2 Beta) Silvertone Preferences.agprefs  and (LR3 Beta) Lightroom 3 Beta Preferences.agprefs files.

On Windows XP, open Windows Explorer and navigate to the Documents and Settings/[Username ]/Application Data/Adobe/Lightroom/Preferences folder, delete the (LR1) Lightroom Preferences.agprefs, (LR2) Lightroom 2 Preferences.agprefs, (LR2 Beta) Silvertone Preferences.agprefs  and (LR3 Beta) Lightroom 3 Beta Preferences.agprefs files.

Note to Windows users, sometimes Windows likes to hide folders from you (for your own protection), so a shortcut to the Preferences folder is to open Lightroom, then go to Edit > Preferences > Presets and click the Show Lightroom Presets Folder button. This will open Windows Explorer to the Lightroom folder (listed in the path above) that holds the Preferences folder.