I purchased a new western digital my passport essential external drive. It keeps powering down/ejecting on it's own. It's the kind that is powered by the USB ports on your computer. I formatted it with fat32 but I also tried Mac formatting and it did not matter. I'm using it on a late 2008 MacBook pro. I tried a different USB cable with no benefit. It shuts down at what seems like random intervals. Any ideas on what this can be? Thanks.