On Thu, 4 Dec 2014, Bill Bogstad wrote:

Unfortunately I don't have performance statistics for hard drive media...  
Which is what I think you're actually seeking...

Yeah, that really is what I want.   Without performance statistics, it
is impossible to
know if I would need to keep copies on one or ten unpowered disks.
Should I copy the data to new "media" every year or every decade?

The real problem is that the technology is changing fast enough that by the time you are able to get "real" stats on something, it's long obsolete. Hard drive technology changes fairly significantly every couple of years, and individual factories make 'minor' changes _very_ frequently (ever had a 'batch' of bad drives? thats because there were changes that affected that 'batch' and not other drives of the same model)

The manufacturers do "accelerated ageing" tests where they try to test in six months if the drive will still work 10 years from now.

Personally I don't trust that testing to be anything more than just a wild guess.

What you really need to do is to have at least a couple of copies, and periodically try and access them. If you fail to access one, then copy another to new media.

David Lang
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