A frequency coincidence detection circuit for detecting frequency edges for each of a plurality of periodic digital signals. A protrusion is positioned on the disc-facing surface along the first side edge and has a protrusion surface. Copper wiring is disposed below a top surface of the first dielectric layer. The identified file records associated with the subdirectory are backed up from the source VM disk file to a target VM disk file, without having to back up remaining files in the source VM disk file.