/For=ForVolumeSpec /On=OnVolumeSpec /MaxSize=MaxSizeSpec - Adds a shadow copy storage association between the volume specified ...

%1 %2 /For=ForVolumeSpec /On=OnVolumeSpec /MaxSize=MaxSizeSpec
    - Adds a shadow copy storage association between the volume specified by 
    ForVolumeSpec and the volume specified by OnVolumeSpec the shadow copy
    storage volume.  Storage for shadow copies of ForVolumeSpec will be 
    stored on OnVolumeSpec.  The maximum space the association may occupy on 
    the shadow copy storage volume is MaxSizeSpec.  If MaxSizeSpec is set to 
    the value UNBOUNDED, the shadow copy storage space will be unlimited. If the 
    maximum number of shadow copy storage associations have already been made, 
    an error is given.  MaxSizeSpec can be specified in bytes or as a percentage 
    of the ForVolumeSpec storage volume.  For byte level specification, 
    MaxSizeSpec must be %3!d!MB or greater and accepts the following suffixes: 
    KB, MB, GB, TB, PB and EB.  Also, B, K, M, G, T, P, and E are acceptable 
    suffixes.  To specify MaxSizeSpec as percentage, use the %% character as the 
    suffix to the numeric value.  If a suffix is not supplied, MaxSizeSpec is in bytes.  

    Example Usage: vssadmin Add ShadowStorage /For=C: /On=D: /MaxSize=900MB
                   vssadmin Add ShadowStorage /For=C: /On=D: /MaxSize=UNBOUNDED
                   vssadmin Add ShadowStorage /For=C: /On=C: /MaxSize=20%%