What is a lost cluster?

Helpline Q&A Directory

Norton Utilities keeps telling me that I've got lost clusters. What are clusters and why do I keep getting them?

Windows manages files by dividing your hard drive into small sections called clusters. Each file uses one or more clusters to store data and Windows tracks all the clusters in use. Each cluster in use is associated with a single file. Windows knows not to allow a file to put new data in clusters currently associated with another file. By contrast, when you delete a file, its clusters become available again.

A lost cluster (also known as a lost file fragment) appears to Windows as in use, but it does not have a file associated with it. Programs such as ScanDisk and Norton Disk Doctor find and fix lost clusters.

What causes lost clusters? Crashes, mostly, by Windows and by applications. If a program crashes while it is creating, copying or enlarging a file, some of its clusters may not be re-marked as available even though the file the clusters are associated with no longer uses or needs them. That leaves clusters marked as in use, but with no files or other clusters pointing to them.

A loose or defective drive cable can also cause lost clusters by creating bad sectors within clusters. If your system and applications don't crash often but you still end up with lost clusters, try removing your PC's case and checking the ribbon cables running between the drives and the motherboard. We know of no way that a bad drive by itself could cause lost clusters, however. In theory, you could lose clusters by defragging your drive while running other applications, although today's defraggers make that unlikely. Still, it is safer not to have any other programs open while you defragment your hard drive.

When a disk scanner finds a lost cluster, it gives you two options: save the contents of the cluster as a file so you can look at it and decide if you need it, or mark the cluster as unused to free up wasted disk space.

The safest option is to save the contents as a file, but we have never found anything worth saving in a lost cluster. They almost always contain extra copies of information that is safely stored elsewhere on the drive. See also tip What is a bad cluster?