The information for the index is not available until the end, since
file order on tape is dynamic depending on what actually works and in
what order it completes.

  Ie: when amanda starts writing it writes a zero filled 32k
  block to the second file and when its finshed doing all the
  work put a file in there containing

Tapes (in general) do not work this way; you can't write at thbe
beginning without trashing the rest.  Now, if you used a DECtape, you
could, since they have a formatting track that is not rewritten
normally, but they are only 184 KW (12 bit words).  (You can even swap
to DECtape - I've seen this and it's amazingly cool, but a bit slow.)
But seriously, I'm not aware of modern tapes with such a formatting
track - DDS certainly doesn't have it.  (If it did, the capacity would
be known precisely, but it would be lower.)  See
http://www.pdp8.net/tu56/tu56.shtml if you are into retrocomputing for
info about this tape drive.

But seriously:

What I would do is to simply read the entire tape, one file at a time,
and store it on a big disk.  If I don't have amanda, it usually means
my backup server is hosed, so I want to get the bits off tape onto a
second place the _very first time_ the head passes over them, since
you never know how many passes you'll get before failure.  I'm just
being paranoid, but this is a Murphy's Law sort of situation.

-- 
        Greg Troxel <gdt AT ir.bbn DOT com>