The LOCKSS system is a tool librarians can use to preserve long-term access to content published on the web. It has three main functions. It collects the content by crawling the pub-lisher's web sites, it distributes the content by acting as a proxy for reader's browsers, and it preserves the content through a cooperative process of damage detection and repair. The system uses the hard disk holding the copy used for access as a preservation medium; the cooperative damage detection and repair mechanism eliminates the need for off-line backups on removable media. We describe the LOCKSS system as an example of the techniques needed to use hard disks as a medium for long-term preservation.