They used RAID6 over 15 drives each, combining those 15 drives via LVM, allowing the loss of 2 drives from the same group without data loss. This is the same configuration Backblaze uses as well, because even though their fancy custom software mirrors all data to different storage pods, it is easier to just replace a drive and rebuild the raid than rebuilding the whole JBOD from the data on other pods.
Think bad power supply, mains spike, cooling failure, fire, flood .....
If you need raid, you also need a separate copy in a separate location.
In contrast, I lose 5-10% of the spinning HDDs I care about every year.
Jeff Atwood made an interesting post on that very subject a while back, though:
http://www.codinghorror.com/blog/2011/05/the-hot-crazy-solid...
Every spot in our solar system is susceptible to some unrecoverable disaster. If you don't want to lose your data, you have to have copies in more than one physical location.
Seriously, I didn't say he was wrong in theory, but focusing so much attention on the highly unlikely to the detriment of the exceedingly common is a drastic distortion of the issue, and a cost/benefit analysis skewed in either direction can have costly repercussions.