julian67
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You're right, there's no substitute for having multiple backups but HDD's do not fail on a uniform schedule even if they are the exact same model running on the same hardware at the same temperature. The chances are ridiculously slim that both drives will die at the same time such that you don't have time to replace the bad hard drive before the 2nd one fails. And of course there's also the added benefit that redundancy is done automatically so there's no need to setup a backup routine, the data is mirrored automatically when it is written.
That said, you're right, there is no substitute for having multiple backups.
Backblaze (an online backup company that broadly speaking uses the same consumer grade disks as you and I) occasionally publishes interesting data on disk longevity, failure rates and so on. See http://blog.backblaze.com/2013/11/12/how-long-do-disk-drives-last/ It is not comfortable reading unless you are organised enough to purchase and discard disks every X number of years (where X is probably 4), and you also have a decent back up regime. On my home desktops and laptops and storage devices I've experienced a number of disk (and more rarely motherboard) failures over the last 15 years aprox. (since owning my first PC) and quickly learned the value of duplication and back up. Most of my data losses can thus be attributed to my own dumb mistakes, or very angry girlfriends smashing the ****** out of my stuff for reasons unrelated to technology or to civilised discourse.
As someone who has, in the past, tried to save money in the wrong places I can attest that disk failures (or mere partition table or file system corruption if you are lucky) also occur because cheaper consumer disk enclosures are as dreadful as their low price suggests. There really is no substitute for backing up your data to good quality disks managed by good quality controllers. Cloud back up is great until the day of reckoning comes and you discover that cloud backup vendors find it easier to refund your subscription and yadayadayada with the bs than to restore your data. Cloud hosting and back up is a bit like the banking system's reserve deposits - very important and real to you and me, but utterly inconsequential and notional to the people who are making profits and don't quite believe the day will ever arrive when payment is required.