This function attaches (mounts) a specified File System Driver (FSD) to a logical disk volume. Function 04h is typically used to force mount the FSD that is formatting a volume. It is called before starting a format operation on a volume. This function also unmounts a current FSD (if any), and forces a mount to the specified FSD.

A flag is set in the OS/2 kernel to indicate that a Begin Format (unmount/mount) was done. DSK_REDETERMINEMEDIA clears this flag. Therefore, Function 02h must be performed before the disk is closed with DosClosed. The volume is then ready for normal I/O service from the newly mounted FSD.