I believe the -a switch is to analyze notto actually defrag.
-a If you want to simply see how fragmented a file or files have become, use the -a switch to have Contig analyze fragmentation.
SOURCE: Contig
C:\Users\johnsmith\Downloads\Contig>contig -a C:\pagefile.sys
Contig v1.6 - Makes files contiguous
Copyright (C) 1998-2010 Mark Russinovich
Sysinternals - [URL="http://www.sysinternals.com"]Windows Sysinternals: Documentation, downloads and additional resources[/URL]
[COLOR=#ff0000][U][B]The process cannot access the file because it is being used by another process.[/B][/U][/COLOR]
No files fragmented.
C:\Users\johnsmith\Downloads\Contig>contig -a C:\windows\windowsupdate.log
Contig v1.6 - Makes files contiguous
Copyright (C) 1998-2010 Mark Russinovich
Sysinternals - www.sysinternals.com
C:\windows\WindowsUpdate.log is in 7 fragments
Summary:
Number of files processed : 1
Average fragmentation : 7 frags/file