Computers consume a lot of power, and power doesn't grow on trees. Okay, carbs grow on trees (apples, pears, etc.), which you can eat for energy so I guess my analogy is lacking, but your computer -- a computer uses a healthy amount of electricity. Apple employs advanced power management to help save energy, the environment, and your power bill. Unfortunately, this can wreak havoc on certain external drives.
Those small little USB drives that are bus-powered (no power cord) are convenient and easy to store, but when used for Time Machine we have seen them get corrupted. Lately we have been suggesting desktop-class hard drives with a dedicated power supply. These are larger units that you plug into the wall. Since they have their own power supply they are free of the computer's power management, and, in our opinion, a better choice for Time Machine Backup with current systems on newer Macs.