Look on about the first of January, 2002, in netbsd-users

  http://mail-index.NetBSD.org/netbsd-users/2002/01/

This thread was discussed at some length.

Wojciech Puchar (I finally found his name again!) posted a script
for this, which was then modified by Jim Breton in what appears to
be the final post on the 7th.  (In a recent post on a similar thread
I incorrectly said that "that Polish guy" (Wojciech) also posted
the modified version, since I couldn't be bothered to search the
archives, and couldn't remember Wojchiech's name.)

The only thing that it uses that isn't part of NetBSD is cdrecord
(from pkgsrc).  It does not build an ISO fs.  It looks like it
should trivially work with any program that can store its output
to stdout (e.g., tar, or yes(1)...(^&).  It's very short and simple.
It doesn't have a GUI, but that shouldn't matter.  It prompts you
for inserting volumes and names them (by paired letters) as it
goes.

It hard-codes a variable, $VOLUME_SIZE, for the capacity of your drive,
so it should adapt easily to DVD.  (But it won't adapt automatically.
You need to set the variable manually.  (^&)

It also assumes that 104 volumes will cover your needs.  If that's not
so, it is trivial to change that, too.

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/