What's a good power supply (volts and Amps) to power a USB hub to use with external HDs

I'm using a 7 port usb hub found here: http://www.ebay.com/itm/AC-Power-Adapter-Cable-for-PC-Laptop-7-Port-USB-3-0-Hub-On-Off-Switches-/331707642029

The power supply it came with was very weak. It could not power on my external 2TB hard drive so I replaced it with a 5VCD 2.5A adapter of my own. It was able to power the hard drives now. Now the issue is that I connect a second 1TB hard drive on that Hub and things get unstable. It works fine if I connect it directly to my laptop. I'm using a raspberry pie which isn't able to power these hardrive on its own.

Hard drive details:
2TB seagate backup plus and 1TB WD blue
both are 2.5" in size and 5400rpm.
The 1TB WD drive uses 5VDC and 0.55A.
I have no idea how much power specs the Seagate one uses.

I think I might need a better power supply to get rid of these issues but I don't know how much volts and amps it needs to be.

Answer

In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component.
— Series and parallel circuits

So, you will need a charger that has the same voltage and sums up the currents (ampere). You can provide more current than the calculated sum, as the attached devices will only consume the current they need. However, DO NOT provide less current than the sum or you could damage the charger and possibly its environment.