Why do companies still use tape drives?

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http://serverfault.com – I found this, but it doesn't really answer my question. Disks even in the 1-2 TB range are much cheaper than tapes, yet I saw that Google uses tape drives for backup. What is their (disks) primary disadvantage? I can imagine some sort of disk "library" that is just a bunch of sockets you can slide a SATA disk right into. (HowTos)