Additionally BackBlaze do and publish a lot of research into hard drive reliability, which is encouraging, google for "backblaze blog" and you will find it.
Finally, I pay $5 a month for unlimited space for one computer and unlimited external drives. (Thats how I have 5TB backed up on it). For the external drives, they needed to be plugged in once every 30 days or they disappear off the backup. If you have left them unplugged for 20 days, back blaze start emailing to you remind you to plug them in for a few minutes.
For me its not an issue anyway as they are plugged in all the time. Finally, I do have a local backup too, but only this week, I found that human error on my part 2 weeks ago had deleted 6GB of photos from my drive and my physical backup. Since Backblaze also allows you restore files from within certain time periods as its incremental, I got it back within the hour.
If you want to signup here are two links:
(the above link credits me as a referral, if you don't want to do that just visit http://BackBlaze.com )
Finally, for those unsure, this is not a sponsored post, just an honest opinion based on my expereince.