A method of manufacturing a glass or glass-based non-magnetic substrate for a magnetic or magneto-optical (MO) data/information storage and retrieval medium comprises sequential steps of providing a non-magnetic, glass or glass-based sheet having at least one surface and containing alkali metal ions and/or alkali metal-containing species; treating the sheet to reduce the concentration of the alkali metal ions and/or alkali metal-containing species in the sheet to a predetermined level for a predetermined depth below the at least one surface; and treating the at least one surface to substantially remove therefrom any alkali metal ions and/or alkali metal-containing species resulting from the previous step. Also disclosed are methods for manufacturing magnetic and MO media utilizing the thus-formed substrate and the media obtained thereby.