When you deal with cold storage you have to look at things from a node level, not in global storage size. If your basic unit is a 50GB device instead of a 4TB device, this means that each request you make to recall data has a much smaller footprint.
Let's say that each stored account takes up 1GB of space. That's 50 accounts per BD drive, and 4000 accounts per hard disk. This means that when some dude comes out of jail and tries to access the photo his mom posted on his Facebook wall in 2010, there are 3999 accounts that are pulled out of their coma with it for no reason. On a BD that's only 49.
As long as you partition stuff properly it's unlikely that a single request will span multiple BD drives. You may have to deal with clusters of BD disks and this requires a bit of tuning, but even with the best indexing system in the world you can't power up only part of a hard disk. So BD is a clear winner here, especially if to that footprint issue you add the fact that spinners die quickly when you keep playing with the on/off switch.
Bytes are bytes when you live in a software world. But physical factors and limitations come into play when you deal with storage, and that's why most people with a software background can see WTF where there is instead good engineering.