From wikipedia - Bob Rock produced Metallica's multiplatinum self-titled album (known as The Black Album) in 1991 and subsequent followups Load (1996), ReLoad (1997), Garage Inc. (1998) and S&M (1999). Metallica once again tapped Rock to produce their 2003 album St. Anger. Because the band had no bassist due to the departure of Jason Newsted, Rock played bass guitar on the album. Fans criticized the production, resulting in thousands signing an online petition. In February, 2006, Metallica chose producer Rick Rubin to produce their next album, ending the group's long-time relationship with Rock.