On an SiO2-patterned sapphire substrate, polarity-inverted lateral overgrowth produced N- and Ga-polar GaN on the opening regions and on the SiO2 mask regions, respectively. Ga-polar GaN was found to contain some narrow-stripe-shaped N-polar GaN domains. Wet chemical etching of N-polar GaN domains on the opening regions made this GaN film epitaxially separated from the substrate. The removal of N-polar GaN embedded in Ga-polar GaN on the SiO2 mask, however, resulted in the fragmentation of the GaN thin film. During the subsequent growth on this etched GaN film, the effective suppression of GaN nucleation on the exposed sapphire surface maintained its epitaxial separation from the sapphire substrate, while the fragmented GaN film was recovered to a continuous one. Thus, the removal of the SiO2 mask by a HF solution allowed the GaN thin film to be readily separable from the sapphire substrate. The separation of the GaN thin film demonstrates that it was epitaxially separated from but physically attached to an SiO2-patterned sapphire substrate. Raman spectra of our as-grown sample, which reveal both strain relaxation and the suppression of extended defects via lateral overgrowth, suggest that this epitaxially separated GaN thin film template can be a good alternative to a free-standing GaN thick film.