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We proposed a method to predict the heights of upper and lower air-oil interfaces of the fluid dynamic bearings (FDBs) in the tied shaft of hard disk drives (HDDs) during non-operating condition. We formulated a pressure equation according to capillary pressure, atmospheric pressure, hydrostatic pressure and applied pressure, and a volume equation according to the volumes of the clearance and the injected oil. We solved the linear equations of pressure equation and volume equation simultaneously to calculate the heights of upper and lower air-oil interfaces. Finally, we verified the proposed method by measuring the height of upper air-oil interface. Height of upper air-oil interface increases with the increase of pressure difference because pressure of lower air-oil interface is higher than that of upper air-oil interface. Since the volume of injected oil is constant, height of lower air-oil interface decrease with the increase of pressure difference.