Disconnecting a drive without advising the computer to "Eject" the hardware can cause data loss. Follow the procedure below to do this on Mac OS X
Open a Finder window, and locate the drive in the DEVICES category
In this example, the external drive is called "BACKUP". To remove it, click the small eject symbol to the right of the name. After a few seconds, it will disappear from the list. The drive is now ejected, and can be unplugged.
Another way to do this is to drag the drive's desktop icon to the trash. The trash will turn to an eject symbol. When the mouse button is released, the drive is ready to be unplugged.