There's nothing wrong with the phone or the computer, it's more of an overlooked usability issue. You should be able to select different "USB modes" on your phone. For example, on my droid 3, when it is plugged in to my mac for charging, I can pull down the notification area at the top of the phone screen to reveal the USB mode message - click it and select "Charge Only." By default "USB Mass Storage" is selected, and as a result, every time it is plugged in to charge 3 or 4 items mount automatically. If you select "Charge Only" they will never mount and your issue will never re-occur. If you use the "Motorola Media Link" or other similar software that requires the storage drives to mount for syncing, you can simply switch back to USB Mass Storage and they will mount. Switch back to "Charge Only" before unplugging the USB cable.
If in USB Mass Storage mode, it will re-mount every time you unmount it - so there's no way around the error message other than to disable the mounting completely by putting it into "Charge Only" mode.
It is not documented but should be.
Good luck and have fun!