Sorry Luke can you clarify this information. When you refer to 500 and 800 I assume you refering to Gb rather than mA ? Second not clear on when you refer to one and or two ports, since my Fox T2 only has one USB port.
If you really only have one USB port, you probably have an HD-FOX not an HDR-FOX (one on the back under the Ethernet port, one on the front behind a small flap). I'm not sure where the 800mA figure came from, but the 500mA spec is in the HDR-FOX user manual (click to enlarge):
This is the current that Humax effectively guarantees will be available from the USB port to power an external device. An external disk drive draws current to power the electronics and the motors, and requires more current while starting up than when in continuous operation. The start-up current may exceed the supply capability of the USB port.
Either way, you cannot make a generalised association between power requirement and drive storage capacity. The drive current requirement is a specification the manufacturer of the drive will state.
Supposing that the drive specification says it draws less than 500mA maximum, you still have the problem of demonstrating whether the drive is drawing more current than its specification or the HDR-FOX is supplying less current that its specification.
It is also possible that this 1TB drive is fitted with a USB3 interface. While USB3 has a fall-back mode which is data-compatible with USB2, a USB3 port has a higher supply current specification than USB2 and it is possible that the drive has a USB3 appetite. If the drive works on one HDR-FOX but not another, it is possible that the drive is drawing a little in excess of 500mA worst case and one HDR-FOX is capable of supplying it but the other is not - simply through normal tolerances. One cannot conclude that the HDR-FOX is faulty without further examination.