I believe Glacier is an all-hard-drive solution. Tapes are troublesome because they're not "online" so it's harder to verify data integrity and notice failures.

The drives are spun much slower than a regular drive (I've heard 300rpm) and so they can be crammed into much higher density racks without worrying about heat.

Maybe data is eventually moved to tapes for Glacier; I am privy to no special information.