A chassis design that holds the HD's vertically[1] would half the space; down to 12U instead of 30U, which is slightly easier to stomach. However, that's less than half of Soundcloud's reported 2.5 PB[2], though I have no idea how much of that 2.5 PB consists of downsampled copies which wouldn't need to be stored.

[1] https://www.supermicro.com/products/system/4U/6048/SSG-6048R... [2] https://aws.amazon.com/solutions/case-studies/soundcloud/