S3 Classic: Your file gets replicated on 3 live HDs (and/or HDs backed by RAID arrays—not sure about the internal S3 storage topology).
S3 Infrequent: Your file gets stored on 1 live HD (or single hardware redundancy component) and a copy in Glacier. If your live HD dies, your file will be automatically restored from Glacier to a new HD (but your data may be inaccessible during the automatic re-deploy).
Glacier: offline bluray combined with error correction accessed by robot arms, temporarily restored to live HDs on demand.
Whichever they use, most disks are likely unpowered most of the time. Like the Facebook equivalent where they can only power 1 out of 12 disks at any time.
S3 Classic: Your file gets replicated on 3 live HDs (and/or HDs backed by RAID arrays—not sure about the internal S3 storage topology).
S3 Infrequent: Your file gets stored on 1 live HD (or single hardware redundancy component) and a copy in Glacier. If your live HD dies, your file will be automatically restored from Glacier to a new HD (but your data may be inaccessible during the automatic re-deploy).
Glacier: offline bluray combined with error correction accessed by robot arms, temporarily restored to live HDs on demand.