Nothing authoritative. The links down-thread are almost all Robin Harris speculating about what it might be based on using various back of the envelope calculations. I'm actually pretty amazed that Amazon has been able to keep even minimal details off the web as far as I can tell.

The closest I've seen to a direct statement from Amazon is: " An Amazon statement sent to Ars says only that "Glacier is built from inexpensive commodity hardware components," and is "designed to be hardware-agnostic, so that savings can be captured as Amazon continues to drive down infrastructure costs."" http://arstechnica.com/information-technology/2012/08/for-on...

FWIW, Google's competitive Nearline service almost certainly uses disks:

The difference between Google’s Standard Storage and Nearline Storage slices on its public cloud comes down to a software layer, Tom Kershaw, director of product management for Google Cloud Platform, tells The Platform. We also think there might possibly be extra physical distance between the devices hosting the Nearline Storage and servers residing in the Google datacenters and network links that expose Cloud Platform services to the outside Internet. The Nearline Storage could also be running on slightly older equipment that is heavier on the disk drives but that is not beyond its technical or economic life.

http://www.theplatform.net/2015/03/16/why-the-coldest-storag...