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 a computer. Most mobile drives require approximately 4 watts of energy to run. Most computers produce this, so it isn't usually a problem.
Multiple Drives
It is possible to run multiple bus powered drives, but note the amount of power taken. Two drives takes the power requirement to 8 watts, which few computers can do comfortably. Also, a laptop will quickly have its battery drained, and that can cause an interruption in communication with the hard disk (which can translate to data loss)
Use the FireWire connection if possible, as the amount of available power is usually much greater than that of USB. It doesn't solve the issue of a rapidly draining battery, but it does avoid other power-loss problems that can occur.
Daisy-chaining
A "Daisy-chain" is a computer jargon term for connecting devices in a linear chain, rather than each device going to a central point. Bus-powered devices can be set up in this way, but it is not advised. Doing this places stress on a single port, which may tax the port beyond its specifications (up to causing physical harm.) It is best to not go beyond two devices in a chain in most situations. Contact the computer manufacturer of the machine the drives will connect to for additional guidelines and suggestions.
