RP,
Your experience is strange, as you say the disk is unrecognised when connected directly to the USB port, but sort of works a bit when connected to a non-powered hub.
USB devices are classified either as "Low Power" or "High Power", and use a maximum of 100 or 500 mA respectively.
Since the USB bus can provide a maximum of 500mA divided amongst all devices on the bus, power consumption tends to be a big problem, unless a device (or downstream hub) is self-powered. If you hang a non-powered hub off a USB port, it has to share all the power from its uplink port (which is already sharing the max) to its downlink ports. In theory you can connect 127 devices to a USB Root Hub (but not without external power)!
Note that until the device is recognised by the O/S, it CANNOT receive more than 100mA, so most high power devices require a staged power-up, with a low-power initial configuration stage while the device gets recognised by the system, followed by the full power allocation allowing the device to complete its power-up.
This might explain why your system is able to recognise the device but is unable to supply sufficient power to sustain the reads. Remember, finding the directory listing is just looking at the file allocation table (which may be cached), and may not require full power until you try to stream the files off. But it doesn't explain why it doesn't work with the "raw" port.
Devices that require more than 100mA to configure are going to need either a dual-USB cable (200mA) or some other form of external power supply.
Not all USB implementations are equal, either due to deficiencies in the hardware, BIOS or O/S. There are known problems with power drop (and droop) affecting different chipsets, depending how well (or badly) the circuits have been designed.
Generally, the older the device, the more likely it is not to correctly implement the USB spec. Also, laptops are notorious for suffering power droops and drops - as well as not being able to actually provide full power!
For example, I have a USB external HD that runs fine unpowered off a Compaq laptop, but won't run without external power off an older Toshiba - fortunately, the disk caddy has a useful keyboard/mouse port power adapter.
SD