I concur with what Joe and Paul have said.  Our biggest concern is for the
data once it has been moved to tape, either through migration or through a
backup directly to a tape pool.  That tape is the only place where that data
exists.  In many cases, the files on that tape will remain the active copy for
a long, long time.  I have to feel comfortable that when a machine is backed
up today and then a file restore is requested next fall, that we will be
able to retrieve the data.  We currently do not have a tape robot, so the issue
"operational accidents" is important to us.  In addition, tapes have been known
to go bad spontaniously, for no apparent reason.  When this happens, we need
a way of getting the information back from another source.  It isn't really
important to me that the "twins" be exact duplicates of each other.  As long
as they contain the same data, they don't have to be identical.

Martha