Mobile drives are commonly referred to as "Bus Powered" devices. They don't typically require an external power supply, and are powered by the ports on the computer. Ordinarily this is an acceptable amount of power, however, not all machines are capable of running these devices.
Most mobile drives require approximately 4 watts of energy to run. Most computers produce this, so it isn't a problem. There are exceptions, however. The physical symptom of insufficient power is a soft "clicking" sound. OS behavior can vary.
All mobile drives have a "USB Power Sharing" cable included in the box. If a power problem is suspected, connect this cable to a second USB port on the computer. This will allow the drive to take more power, and can possibly make the difference in allowing it to work. Also, ensure the drive is not connected to a USB Hub of any kind. Connect all bus-powered devices directly to the computer USB ports.