This paper covers a number of issues that must be considered when evaluating the use of disk in a backup architecture. While it has been easy for a wide variety of vendors to tout support for using their disk in any backup environment, it is clear that there are specific circumstances that must be approached with caution before moving these kinds of products into production environments. Given that no two backup environments are exactly alike, something that sounds as simple as "use disk instead of tape, it's the same thing--only faster" can rapidly devolve into a situation that is unexpected and that no one has time to spend cycles repairing.