I consider optical media as very short-term storage. It's only ideal for portable and disposable use, and is in fact the functional equivalent of the floppy disk from a decade ago.

Consider the time spent burning just 40GB of data (equivalent to roughly 5 dual-layer DVDs)... It WILL take a while. Then if you are the careful one, you'd want to verify the data after it has been recorded, eating up several more minutes of your time. And THEN, you have to be there, shuffling and changing disks, etc.

Now, the context of this thread is probably directed towards individual users thinking of ways to preserve their personal data (pictures, videos, digital documents, etc.), and not corporations with the financial capacity to buy the best backup solution. So that kinda puts "temp-controlled vaults" for personal use out of the question. :D

Anyway, HDD technology have matured dramatically compared to MFM/RLL drives of yesteryears. So much so that the stories about the two feared enemy of the HDD, the magnet and impact/shock, are usually put out of proportion. It is true that a sufficiently powerful level of magnetism placed in just the right way will corrupt data inside a HDD, but it has to be strong enough or the proximity close enough before any harm is done. One of the strongest magnets known to man, neodymium, is being used INSIDE the hard disk in conjunction with a voice coil to move the head actuator. I used to collect these from defunct hard drives and was stupid enough to get pinched by a pair of these magnets. They're really strong! But yeah, for those in the know, they are installed in a way that the magnetic flux are practically facing away from the platter, so the residual magnetic field is way below the threshold of the platter.

But I do agree that HDDs, still being mechanical, are bound to fail eventually. That's why it is vital to keep copies of your backup on two separate HDDs. It's statistically improbable that both HDDs will die at the same time, even if they were assembled on the same day by the same person. Even so, it's so much more easier to work with HDDs. Data transfer is fast and reliable, the capacity is scalable, it's relatively cheap, and it's available anywhere. The convenience and performance alone far outweighs any if its shortcomings.