There is one more advantage to optical media — it is the only commonly available format that is (or should be) entirely waterproof. Optical discs should retain data even after a flood — clean the mildew off and the disc reads fine. (Again, quality is important here, since poor quality discs might not be properly sealed, leading to "laser rot" even if not exposed to water.) This may or may not be relevant to your risk model, and 3TB is a very large amount of data.

I have so far avoided the "unreadable several years later" problem by using good quality media from reputable manufacturers that I buy when the stores put it on sale (usually almost half-off if I am patient). Since I buy the blanks when they are on sale, I have a significant personal stock that I slowly rotate, and I suspect (and hope) that the blanks that will go bad will go bad before I get around to putting data on them. So far, this strategy has worked and I have yet to retrieve a disc from storage and find it unreadable, although I have had many discs fail verification immediately after writing them. Always read back an optical disc immediately after writing it — do not expect the drive to notice that the blank is bad while it is busy writing data.

It is probably best to rank by importance (favoring more copies on lower-density media) and bulk (requiring fewer copies on higher-density media). This means the data with higher bulk-to-importance ratios (like high-def video) is exposed to greater risk of loss, but one partial mitigation is to store lower-resolution more-compressed copies of those videos in lower-density "bands" in your archive. I still use CD-Rs for some backups, even though I mostly use DVDs now. (But I do not have a significant collection of video.) So you might have full high-def video stored only on spinning hard disks, but lower-resolution "better than nothing" transcoded copies on DVDs or BDs.

By now, you have probably learned better than to consider SSDs as valid backup storage. :-) (But they could still be a 3rd technology holding a 4th copy.)

In reply to Re^2: Adding cols to 3d arrays - syntax by jcb
in thread Adding cols to 3d arrays - syntax by peterrowse

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