My 500g hard drive only show 250g - Western Digital Caviar 500GB Internal SATA Hard Drive

SOURCE: WD Hard drive no longer works

This is mostly a power issue, but if you use a hub it may be latency as well. Some USB ports limit power to 400-500ma per port, the Passport Essential needs a bit more than 1000ma and won't work at all with these computers. Other USB ports/hubs limit the total power available for all devices combined... this is why, in your case, you can use one drive, but not both.

FYI, higher speed devices also tend to need shorter cables due to latency. (Light & electricity travels about 1 foot, or 30 cm, in a billionth of a second; this limits the size of high speed devices. In layman's terms, latency is the Brontosaurus effect, since the head takes a while to talk to the tail.)

To boost power, Western Digital recommends using a 22" USB 2.0 Power Booster Cable (WDCA029RNN) to combine the power of two USB ports into the one Passport Essential USB port--it is also a bit longer than the original 18 inch cable. For all EXCEPT the black Passport Essential drive, you may also the WD Passport™ Power Supply Adapter (WDPS021 for the US voltage version). See Western Digital knowledge base ID 1296 for more on this issue.

Hope this helps!

SOURCE: WD 500g hard drive computer cant read it

Is your hard drive power-up?
Do you hear your hard drive just spinning?

If so, your PCB may went bad. If you hear the hard drive clicking then your PCB or the heads went bad. If you need the data, you might have to ask the data recovery guys to get them back for you or find the PCB on Ebay.

If you don't care about your data, send it back for the warranty replacement