Obviously, yanking out a drive while it's being written to could corrupt the data. However, even if the drive isn't actively being written to, you could still corrupt the data. By default, most operating systems use what's called write caching to get better performance out of your computer. When you write a file to another drive—like a flash drive—the OS waits to actually perform those actions until it has a number of requests to fulfill, and then it fulfills them all at once (this is more common when writing small files). When you hit that eject button, it tells your OS to flush the cache—that is, make sure all pending actions have been performed—so you can safely unplug the drive without any data corruption.