R&B superstar Usher reached a notable milestone this week as his 2004 album "Confessions" surpassed 10 million in US album sales, becoming just the 19th album ever to achieve those lofty numbers since Nielsen SoundScan began tracking sales in 1991.  The last album to reach this level was Eminem's "The Eminem Show," which crossed the threshold in October 2011.  The biggest-selling album, set in the SoundScan era, is Metallica's self-titled 1991 classic, also known as "The Black Album," which has sold 15.8 million copies in the US.