Assume you have a large one terrabyte database or several databases adding up to a terrabyte. With today's (2005) technology, it takes around 20 hours to back it up to tape. To speed it up, you would have to configure more tape drives. Multiple backup devices e.g. \\.\TAPE0, \\.\TAPE1, \\.\TAPE2 and \\.\TAPE3, work in parallel thus reducing backup time to 5 hours in this example.

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How to: Back Up to Tape or Disk (SQL Server Management Studio)