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One of the existing methods for measuring the linear thermal and hygroscopic expansion coefficients for PET (polyethylene terephthalate, mylar) floppy disk substrate material uses two adjacent tracks recorded on media clamped in a disk drive. Recording is done at different environmental conditions for each track and then the distance between these tracks is measured along different radii. The coefficients and associated anisotropy are estimated from this and the environmental data. In this paper simple expressions are obtained for these estimates and it is shown how to account for the correction due to the media clamping. An error analysis of this method is given. It is shown that the determination of coefficients by this method for the current drives and media is not practical because it can be used only for the anisotropy estimation, the error involved in the estimation of the coefficients themselves being substantial.