Why would you compare the cost of an SSD in this case?

You can get a good 1TB HDD for $50.

A mechanical drive is more likely to fail if exposed to shock or magnetic fields; unless its safely encased in a rack in a datacenter, its not an acceptable form of data storage.

Citation needed. You don't need for the drive to never ever go bad you just need for primary and backup not to fail in the time window required to replace the drive. Most of planet earth relies primarily on magnetic storage including people outside the data center.

I need citations that mechanical drives will fail from shock or magnetic fields? I'm not going to waste my time googling references for that.

> Most of planet earth relies primarily on magnetic storage including people outside the data center.

Right. And I'm arguing SSDs are sturdier in that regard when not safe in a chassis somewhere. SSDs can withstand higher shock forces, have no glass platters or mechanical arms that can contact each other, and are not affected by magnetic fields.

It takes quite a bit of shock force [1] and a pretty insane magnetic field [2] to damage rust.

Also you could buy 2 6tb hdds [3] for the price of that single ssd and mirror them for another level of redundancy, so in my book they come out way ahead of ssds and cloud storage for backup/archival purposes.

1: http://www.tomshardware.com/reviews/a-sturdy-companion,758-2...

2: http://superuser.com/questions/568336/what-is-the-tolerable-...

3: https://smile.amazon.com/Marshal-7200RPM-Internal-MAL36000NS...

If you use a filesystem that checksums you will know if the drive is likely bad and wont lose data unless local and backup go bad within the window required to get a replacement for whichever died. See zfs.

If your proposed backup solution is only used for periodic backups and isn't in constant heavy use 2 $50 1tb drives in raid1 would be quite secure against data loss and last years and for a bonus would be unlikely to be compromised by hackers or mined by the nsa for signs you are a terrorist.

If you project replacing the drives every 5 years your annual cost could be $20 and your up front cost no higher than dropbox. Its even entirely likely that you could get by with a single drive for a annual cost of $10.