Tape vs Disk For Backup and Data Recovery

Data backup and archiving is sometimes a waking nightmare, how best to balance the need for instant access against the equally important needs for security and and to be able to recovery data when required? As many IT professionals can testify, few people notice when the systems are running well but everyone right up to the top of an organisation will quickly leap into the fray when there is a problem, and if there is data loss it gets worse, with the possibility of external regulators becoming involved. What is the best system to guarantee that data can be recovered when it is needed? Are disk based easy access systems a better option than tapes and tape libraries, or are the more traditional data backup and data recovery methods a better bet for long term data security? Each technology has its exponents and its detractors. Tape is seen by many as slow and inflexible whereas disk based systems give a convenient, easy to operate, backup system with the ability to add on extra features such as single instance storage and de-duplication that require a dynamic filing system. Then there is current low cost of hard disks, a 1.5TB disk does not cost that much more than a 1.6TB LTO 4 tape, and the tape capacity is based upon average data compressibility, the native capacity is 800GB, so disk is not the expensive option any more. Does this mean that tape is going the way of the Dodo and that the future is disk based? The question to ask is "what is the purpose of our backup and archival system".