Official comment
Yeah, the problem is that your username in the preferences file is causing rekordbox to try and use a location that doesn't exist. Try the following:
First things first, close rekordbox.
Next, you need know what your username is.
Once you have that, open a Finder window and go to the menu Go > Go to Folder... and in that window, copy/paste this:
~/Library/Application Support/Pioneer/rekordbox/
Right-click on the rekordbox3.settings file and select Open with... and select Text Edit.
Before you make any changes, open the Text Edit Preferences and make sure you disable the Smart Quotes, then close the preferences.
Next, use the Find to locate these lines:
<VALUE name="pioneerDirectory"
<VALUE name="masterDbDirectory"
<VALUE name="itunesLibraryFile"
<VALUE name="bridgeImportedLibraryFile"
<VALUE name="SamplerSaveDirectory"
<VALUE name="RecordSaveDirectory"
If they don't exist, that's fine - you should have the top two, at the very least.
In that line, it will have a val and the data for that will be the path to your OLD user's files; you need to update that. For example:
<VALUE name="pioneerDirectory" val="/Users/PulseOLD/Library/Pioneer/rekordbox"/>
would become
<VALUE name="pioneerDirectory" val="/Users/PulseNEW/Library/Pioneer/rekordbox"/>
Once you've made the changes, save the file, then re-open rekordbox. You should be good to go!
