We are as an engineering file contractually obliged to send out all documentation in 3 forms; (1) ready use by email as PDF or raw drawing/document; (2) Final documentation as DVD (multiple copies to different addresses of all documents) and correspondence during contract; (3) One hard copy of all documents in "2".
These form in 3 and one of the 2 copies the necessary 40 year expected life resource for any death or injury requirement and subsequent legal inquiry. (Our plant makes/ handles some very nasty chemicals, and safety back ups of all documentation go to insurers , classification authorities and at least one legal copy to our own and customers archives.
We go through a lot of DVDs.
Reason for DVDs , not alterable after burning. Reasonable shelf life. However do keep one of two external and internal DVD readers/ burners on the computers.
Hard drives (electro-mechanical) are not as resilient to a start up after long term idle storage (bearings1).