Recommended shutdown procedure for Mac systems with a USB 2.0 or FireWire drive
In some cases, shutting down the system without properly unmounting the drive results in data corruption. If the OS stops the device while data remains in the drive cache, data can be lost or file system damage can occur. To avoid possible data corruption, it’s recommended that FireWire and USB drives be properly unmounted from the Mac OS prior to turning off the system.
To safely disconnect the drive, simply drag the drive icon to the trash. This will assure that all data is properly cleared from the drive cache before the drive is removed. At this point the FireWire or USB data cable can be safely removed and the drive and Mac system can be powered off.