Syntax: DiskMgkC /H - Display Help

Syntax: DiskMgkC /A - Action > {DefragQuick|DefragComplete|DefragAdvanced|Stop|Monitor|List}

Syntax: DiskMgkC /A - Drive > [/D Driveletter|Guid>]

* Additional 'Action' parameters will be supported in future releases

Also....

If you don't have a drive letter assigned you can use the guid of the drive, for example:

M:\ \\?\Volume{f64d4030-86be-11dc-97ad-0013d4b14709}\