&gt; Is there any reason for them to stay with 5.25 or 3.5 inch design for a datacenter?<p>&gt; Why not go back to 8 inch for massive surface area? Or is that too much mass to spin.<p>One problem AFAIK is that larger disks wobble too much, more the faster they spin. Which is why if you break open a 3.5&quot; 15k drive you&#x27;ll see that the platters are more like 2.5&quot;, and a lot of wasted space. For 7.2k you can use bigger platters, but how much bigger? I doubt 8&quot; is feasible.<p>Of course you can reduce the rpm further, but that has downsides too.