Yes, glass was the typical substrate used in small HDDs, even in many of the 2.5" HDDs, e.g. in all the 2.5" HDDs that I had. It is easier to ensure that glass

Yes, glass was the typical substrate used in small HDDs, even in many of the 2.5" HDDs, e.g. in all the 2.5" HDDs that I had. It is easier to ensure that glass substrates are perfectly plane and without any surface defects than for substrates made of aluminum alloy. In 3.5" disks the risk of shattering becomes too great, so aluminum alloy is preferred.