Powering 64 SATA HDD from Single ATX Power Supply
I'm not sure what hard drives you are using, but I'd seriously doubt that you would exceed the +5V rating of any reasonable power supply.

Here is a list of Corsair power supplies with +5V rating of 125-180W

A reasonable Seagate Barracuda range drive draws less than 300mA from the +5V line (1.5W) and 2A peak from the +12V line. 

It seems you would not ever exceed the +5V line capability since for the drive above you could support at least 80 drives.