- Recent technological advances in magnetic storage suggest the feasibility of extremely high-density magnetic recording up to 1 terabit per square inch (1 Tbit = 1012 bits) areal densities. Modeling indicates that approximately 3 nanometers (nm) of physical head-disk spacing is required for such high recording densities. When the recording slider is flying at such ultra low spacing over a high-speed rotating disk, it is experiencing disturbances from various different sources and of a wide frequency range. These disturbances may cause the recording slider to vibrate significantly, a condition that is known as fly-height modulation (FHM), which may result in data loss. A significant source of excitation is from the surface irregularities of the rotating disk and is termed dynamic microwaviness. The term dynamic microwaviness has been introduced recently to differentiate from regular topographical microwaviness that is measured statically. In this paper, the procedure for making reliable dynamic microwaviness measurements of disk media used in hard disk drive (HDD) systems is described. Furthermore, such measurements are made on different magnetic disks that are intended for extremely high recording densities using non-contact laser vibrometry. The source of the dynamic microwaviness and its interaction with system dynamics are also investigated.