Hi,
I have a Humax 7510, I've tried both an USB 2.0 and USB 3.0 1TB external portable HDD and although the lights comes on, the 7510 can't see the drive. One of the drives makes a very loud clicking noise. I'm assuming the USB port is not generating enough power for the HDD. So what is the limitations with what portable drives can be used ... is it only 500GB and under? Or can it only use external HDD that are self powered?
It's common for USB ports on PVRs to only supply the maximum current that the USB2.0 standard mandates: 500mA. It is very common for USB-powered devices to use more than 500mA, commonly 900mA, though it's become very hard to find that information from the manufacturers' "specifications".
I'm not sure what the Humax's USB current limit is. You may find it in the documentation.
Page 62 of the Humax HDR-7500T manual states it provides 500mA per USB port. I'd suspect that the 7510 would be the same.
Many USB HDDs operate at or below 500mA but need much more than that to start up.