Official comment
So long as you haven't really altered the folder structure and the only real change is the drive letter / path, you could do one of two things...
1) If in Windows, assign the drive letter for that new drive to be the same as what the old drive used. rekordbox should automatically pick them up as if nothing had changed.
2) Perform a mass-relocation. Provided it's just the drive letter that has changed, it should only take a few minutes and just a couple of clicks to do this.