To disconnect your Palm Pre from your computer, you need to make sure the software has stopped running on your computer. To do this, find the icon for your connected hardware in your computer's system tray. (It will look like a very small USB connector with a green circle and white check mark next to it.)
Right click on the icon and then select "Safely Remove Hardware."
On the menu that appears, highlight "USB Mass Storage Device" and click "Stop."
A pop-up menu lists all of the USB storage devices connected to your computer. Select "Palm Pre USB Device" and click "OK."
A pop-up notification will tell you when it's safe to disconnect your phone. When you see that message, go ahead and do so.