You'd need 3 pairs of 6TB hard disks in 3 data centers to match the reliability that S3 provides. So at least 6 HDDs, 1800$ of H/W and the monthly hosting cost

You'd need 3 pairs of 6TB hard disks in 3 data centers to match the reliability that S3 provides. So at least 6 HDDs, 1800$ of H/W and the monthly hosting costs for the 3 DCs and presumably 3 server chassis... Plus software to keep them in sync and you need to deal with the inevitable failures that will happen eventually. Amazon guarantees 11 9's worth of durability (99.999999999%). Backblaze sees between 2.41% and 7.77% annual failure rate on 6TB HDDs (source: https://www.backblaze.com/blog/best-hard-drive/ https://www.backblaze.com/blog/best-hard-drive/) and they're the only ones publishing numbers like these over sensible number of HDDs.