400 gigabytes is way too much data to transmit offsite. It is far cheaper and dramatically faster to carry an Ultrium tape offsite. Clearly, the cost-benefit ratio depends on how much data you have, but asking "Why would you still bother with tape" reveals either hubris or ignorance, or both.
Furthermore, the tapes give me a far deeper history than you are likely to have with online backup.
Your disk-to-disk strategy is great for the most common restore request; a user that has inadvertently destroyed her own file. But a worm, virus or logic-bomb may very well wipe out your disk-to-disk strategy, and then where are you?
You can reduce the risk of online backup vulnerability if the host machine uses a different operating system as compared to the OS of the servers being backed up. Also, the shares by which the backup server reads the master files ought to be read-only, so a virus in the backup server cannot infect the primary server.
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