If installed only on a file, an MFS will not be able to trap most calls that update the media map. However, if the MFS is installed on a volume as a whole, the MFS will be called for almost all filing system functions, including those that initialize and update the media map.
If able to trap media calls, the MFS can be used for such functions as volume security. For example, a volume-based MFS can prevent users from creating or deleting files without proper access rights. The calls can be disabled by setting the status argument to false and returning without calling the next filing system.
Calls that are available only if the MFS is installed on a volume are:
For these calls, information passed in the HANDLE argument is the handle of the media map. This handle is used in virtually the same manner as a file handle.
