It's also very beneficial to spread data across different drive manufacturers. If, for example, a particular HGST model starts to have massive failure problems

It's also very beneficial to spread data across different drive manufacturers. If, for example, a particular HGST model starts to have massive failure problems in a year (massive at this scale would be like 20% failure rates), well, at least you still have a healthy population of Seagate/WD drives where the data on the HGST drives is also replicated to. I wouldn't be remotely shocked if the Backblaze software takes drive model into account in data placement. In addition to that, it makes sense from a purchasing perspective to spread your purchases around to many vendors. Make sure that Hitachi knows that they're not irreplaceable if they don't give you a good enough price you'll just buy more Seagate drives. Make Seagate always be worried that if you shifted enough purchasing to HGST they'll cut you a really good deal and potentially undercut Seagate.