Do I have to have a powered USB hard drive or can it be USB powered?

I have already filled my local disc so am now looking into expansion and I have a USB powered hdd but I recall reading somewhere that any drives plugged into the hub needed to be externally powered.  Is this correct?

Thanks in advance

You can use any portable or desktop drive with the hub, in my case I got the device connect to a 1 TB Passport drive.