The ROM filesystem is initialized as the ROM modules are found and initialized, then once the init process gets to your hard disk driver, the hard disk driver loads any filesystems present in the RDB from the RDB into RAM.
If your RDB contains a filesystem with the same dostype as the ROM filesystem, but with a newer version number, it will supersede the one in ROM.
If the ROM filesystem has a larger version, the RDB filesystem shouldn't be used, but this might not always be the case depending on the hard disk driver.
