Almost 13,000 each are Seagate and Hitachi drives, almost 3000 Western Digital drives and a too small for statistical reporting smattering of Toshiba and Samsung drives.
One cool thing: Backblaze buys drives the way you and I do: they get the cheapest drives that will work. Their workload is almost hundred percent write. Because they spread the incoming writes over several drives their workload isn't very performance intensive either."
Link to Original Source