/Provider=ProviderNameOrID /For=ForVolumeSpec /On=OnVolumeSpec /MaxSize=MaxSizeSpec - Adds a shadow copy storage association ...

%1 %2 [/Provider=ProviderNameOrID] /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 1MB 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.  
    If a suffix is not supplied, MaxSizeSpec is in bytes.  To specify MaxSizeSpec 
    as percentage, use the %% character as the suffix to the numeric value.  If the 
    provider is the supplied Microsoft provider, MaxSizeSpec must be 100MB or greater.  
    - The provider ID can be obtained by using the List Providers command 
    When entering a Provider ID, it must be in the following format:
       {XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX} 
    where the X's are hexadecimal characters.

    Example Usage: vssadmin Add ShadowStorage 
                            /Provider={b5946137-7b9f-4925-af80-51abd60b20d5}
                            /For=C: /On=D: /MaxSize=900MB