When you disconnect your external drive from your computer, do you click on the "Safely Remove Hardware" icon on your toolbar..?

I always do that to make sure that the computer is really finished reading or writing to the external drive.

The following info gives the reasons why:
An External Drive has Moving Parts it is a Mechanical HDD that needs it's heads parked before you unplug it.

With any Mechanical Drive you should always use the Safely Remove Option to Park the Heads and then when you are prompted that it's safe to remove the drive unplug it. With Windows that is always on the Start Bar near the clock though the icon does vary between different versions of Windows.

The reason being that once the heads are parked there is no possibility of the heads touching the moving platters as the power drops down and the heads attempt to park themselves. If you do not use the Safely Remove Option it's just a matter of time till you get a Partition Table Corruption rendering all data on that drive unreadable.With memory Drives you should still use the Safely Remove Option to prevent any corruption from occurring though this is less important. The reason to use the Safely Remove Option is that the device may be undergoing a write when you unplug it that is not related to something you have wanted written to the device but something that the OS needs to do. If you unplug during a write operation you will get Data Corruption and possibly corrupt the Partition Tables of the device.