ITProPortal has a good writeup on Amazon Glacier technology: tape? cheap disks they power down? It’s more than just a post filled with wild speculation because it includes informed reasoning on the current state of the art for each of the candidate technologies behind Glacier:
…of all the services offered by AWS, none have fuelled the same level of speculation and interest as Amazon’s Glacier. Though the service is well-known and widely-used in enterprise, no one knows exactly what’s behind it.
Amazon has retained a thick veil of secrecy around its most mysterious web service. The Seattle-based company has always kept the processes behind its services fairly quiet, but the omerta surrounding Glacier has been especially strict, leaving experts in the tech community perplexed about what Amazon could be hiding.
TL;DR: It might be old-fashioned robot tape libraries; it might be cheap disks they fill up then turn off until they need them for retrieval; it might be some clever hybrid of the two.