The option "-b" is not 'documented, but is used by the OS I think every 2 or 3 days. (However it sees fit when the OS ...) to do it only when the system but is based on data from the Prefetcher, so if by chance the prefetch 'was disabled or if the PC has not yet had a being at rest ( "On Idle") for enough time, this process is not 'yet ever initiated and the defrag, not finding the registry key that says when he made the last optimization boot files, which recalls that mistake. the key is this HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Prefetcher\\LastDiskLayoutTimeString However I solved the problem by running this command and restarting rundll32.exe advapi32.dll,ProcessIdleTasks