To add some other examples to Margaret's comments about technology
from our holdings at NASM (WOOP! WOOP! STANDARD DISCLAIMER ALERT!
STANDARD DISCLAIMER ALERT! WOOP! WOOP! - These statements and opinions
are mine alone and should not be taken as representing those of the
National Air and Space Museum, Smithsonian Institution, or the U.S.
Government)

1) We have a largish collection of audio tapes, gathered over a large
number of years - both cassettes and reel-to-reel. This collection is a
source of any number of problems (I'll leave legal issues aside). From a
preservation standpoint the various *materials* may be relatively
stable, but the combinations is are not, particularly as an information
storage media.
  The different reactions of various materials to environmental changes
and the variety of aging reactions can lead to the tape itself becoming
unusable (delamination of the magnetic media from the carrier -
particularly for acetate-carrier tapes). The information on the magnetic
media is intact, but you can't read it.
  On the other hand we have a box full tapes that may or may not have
information on them. In some cases the information was recorded
*backwards.* Other tapes have documentation (on the tape case) but, when
played, the tape sounds blank. Are they blank, or were they recorded
with a non-standard write head arrangement?
  In either case, the information, if it still exists, is lost because
we have no way to retrieve it. This situation is the audio tape parallel
to Margaret's 5 1/4 floppy problem - you've the data but not the
technology to retrieve it (and what if they had turned out to be Apple 5
1/4 floppies?)

2) Here at NASM we have had a videodisc program in place for the last
decade+ (six discs available currently, two more nearing completion) in
which we have placed photos from our collections onto laser videodiscs -
100,000 images per disc. These are 12" laser discs, not CD ROMS
(although our videodisc unit is looking into CD ROM production). They do
not have any "data" component, just the encoded images arranged one
after the other like a movie (which, in fact, is where the system
originated). When you want to see an image, you have to have the disc
player go to the frame number for the image.
  The discs, however, are not the record copies of the images. If the
disc fails (as the first pressing of the first disc did...) we haven't
lost anything. The discs are mastered from a 35mm film, which was shot
from the original photos. The photos are the record copy (still on file
here) while the 35mm film is the duplication master (in cold storage).
If the technology changes, we can remaster the discs from the film. If
necessary, we can pull the original images.
  The difference here is that the digital end of the system is simply a
reference tool. If the technology fails all we loose is convenient
access, not the information itself.

And as for the question of "what happens if you loose the negatives?" -
the whole point of being a museum/archives/library is to *not* loose
things. Right? Well, unless we want to loose them...

Paul

Paul Silbermann                  National Air and Space Museum
[log in to unmask]             Archives Division
**************These opinions are mine, not NASM's**************