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So what would you guys say is the best file type, media format and
media type to use if I want them to be easily accessible for decades?

Consensus among the digital wise men is that no CD or DVD is truly archival and
that it's best to rewrite the data periodically to the current "best storage
solution", preferably in duplicate or triplicate.

Most wise guys are using external hard drives now that base prices are around a
buck a gigabyte, with occasional sales down to 50 cents/GB. Be prepared to
rewrite to other technologies as things change.