Tier 4 once was the simplest of all tiers — it was just tape. Tape drives are not very forgiving if you can’t deliver data to them fast enough. Enter disk-to-disk backup, which has helped most backup strategies, actually has made the tier itself more complex. The challenge with disk backup is that it doesn’t have the portability of tape, says George Crump. There are so many nuances to these different tiers, and more come to market every day, Crump says.”]
Source: https://www.darkreading.com/database-security/tier-4-the-end-of-the-trail-of-tiers