Frequently Asked Question - Why does Defrag return the error "Unable to defragment hard drive"?

Why does Defrag return the error "Unable to defragment hard drive"?

Defrag is a utility provided by Microsoft in the Windows operating system. A help page is available that lists Microsoft knowledgebase articles regarding the use of Defrag at
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q286263