It looks like HGST is the global winner in front of Toshiba, Seagate and WD, in terms of hard drive reliability over time, according to th elatest data released by Blackblaze. The company is frequently providing statistics related to the hard disk failures reported from its datacenter. As of the end of Q3 2015, there were 50,228 drives spinning in the Backblaze datacenter. Subtracting boot drives, drive models with less than 45 drives and drives in testing systems, the company published data on 49,056 hard drives spread across 26 different models, varying from 1.0TB to 8.0TB in size.

Backblaze has also computed the average failure rate for all periods for all drives at 4.81%.

Here are some comments from the company:

• The Western Digital 1TB drives in use are nearly 6 years old on average. There are several drives with nearly 7 years of service. It wasn’t until 2015 that the failure rate rose above the annual average for all drives. This makes sense given the "bathtub" curve of drive failure where drives over 4 years start to fail at a higher rate. Still the WD 1TB drives have performed well for a long time.
• Nearly all of the 1TB and 1.5TB drives were installed in Storage Pod 1.0 chassis. Yet, these two sizes have very different failure rates. Nearly all of the 2TB and 3TB drives were installed in 2.0 chassis. Yet, these two drive sizes have very different failure rates.
  
• Always consider the number of drives (Max # in Service) when looking at the failure rate. For example, the 1.5TB Seagate Barracuda Green drive has a failure rate of 130.9%, but that is based on only 51 drives. Backblaze tested these Seagate drives in one Storage Pod in their environment and they were not a good fit. In general, Backblaze found it takes at least 6 Storage Pods (270 drives) worth of drives to get good sense of how a given drive will perform in our environment.
  
• 4TB drives, regardless of their manufacturer, are performing well. The 2.10% overall failure rate means that over the course of a year, Backblaze has to replace only one drive in a Storage Pod filled with these drives. In other words, on average, a pod comes down for maintenance once a year due to drive failure. The math: 2% is 1 out of 50. There are 45 drives in a pod, so about once a year, one of those 45 drives, on average, will fail.
• 6TB drives, especially the Seagate drives, are also performing well, on par with the 4TB drives so far. The 6TB drives give us 270TB Storage Pods, giving Backblaze 50% more storage at the same overall cost per GB.
• The 5TB and 8TB drives are performing well, but Backblaze only has 45 of each in testing, not enough to feel confident in the numbers yet as can be seen in the confidence interval (low rate/high rate) of these drives.
  

Below is the chart of failure percentages by manufacturer. This is for all the drives in this analysis for the years noted: