Backblaze may not know because they are "a company that keeps more than 25,000 disk drives spinning all the time". After 3-5 years, you'd better have a back-up of a drive you choose to spin down. Every drive I've lost (in the last 10-15 years and ignoring two failed controllers subject to a close lightning strike) failed to start back up when I had powered the machine off for maintenance.
I've bought perhaps 50 drives in the past 20 years, and maybe 10 of them died, the others mostly becoming obsolescent. I only started taking serious logs about 6 years ago.
Drives have died for me both in 24/7 powered systems and through power cycles. Drives have reported intermittent failures for many months, but still lived for years without any actual data loss. The oldest drive I still have spinning is a 200G IDE containing the OS for my old OpenSolaris zfs NAS; must be getting on for 9 years.
I advise having a back-up of every drive you own, preferably two. I built a new NAS last week, 12x 4G drives in raidz2 configuration; with zfs snapshots, it fulfills 2 of the 3 requirements for backup (redundancy and versioning), while I use CrashPlan for cloud backup (distribution, the third requirement). Nice thing about CrashPlan is my PCs can back up to my NAS as well, so restores are nice and quick, pulling from the internet is a last resort.
The one thing to know about cheap consumer cloud backup solutions like CrashPlan and Backblaze is that they only have one copy of your data. So if their RAID array where your data is stored dies and cannot be rebuilt, it's all gone. You can Google for a few disaster stories about both companies.
Like I said, they are only one third of my backup strategy. My house burning down, or someone breaking in and going into my attic to steal my 30kg 4U server, should be the only two realistic scenarios in which I will need to rely on CrashPlan.
I do same, but in reverse. I use cloud servers, with versioning backups, but still beam additonal backups back to office. Just to survive total data center destruction disaster.
Backblaze doesn't use RAID, they use blob-level replication. I'm sure they can lose data, the question is how likely they are to lose data simultaneous with you losing your HD and local backup.
Ideally in most circumstances you should have at least one cold (non-raid, non-connected) backup of all data, and an offsite one. More being much better.
