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Why do they buy such weird proportions of each type of harddrive? Do they just use anything they can get their hands on? If the purpose was actually to test drive failure rates wouldn't you want to buy similar amounts of every drive?

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Drive failure rates can be computed across different batch sizes, provide the batches are big enough to provide meaningful numbers. There's no need to have the batches be exactly the same size to get useful and accurate data.

Sure, but a drive count of 45 is not very reliable.

a pod was 45 drives, so everything they do was in pod units. they would want at lest 45-90 of a drive to have a complete set.

pods are 60 drives per unit now.

They typically purchase the best $ per MB. Even "shucking" external hard drives bought at Costco after Thailand experienced a flood that took offline several hard drive manufacturing plants. [1]

[1] https://www.backblaze.com/blog/backblaze_drive_farming/

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