Dragging and dropping a file to an external disk in OS X creates a copy in the external disk but leaves the original on your local hard drive. Sometimes however, you want to move the file to the external disk and delete the local file at the same time, for example, when you archive your files to a Firewire drive. To copy a file to an external disk and delete the original at the same time, simply hold down the Command key (⌘) as you drag and drop the file. Easy peasy.

via Apple Pro Tips

Aside: I’ve heard people calling the Command key a lot of wrong names. ‘Apple key’ is still find, but please, don’t call it the ‘flower key’!