It’s rather easy. By specs, an USB port has to deliver 500mA @5V if demanded by the device. A slim optical drive needs typically up to 1500mA which is triple of the power an USB port has to deliver by spec.
What can happen due to this overload:
[li]unreliable operation [/li][li]device does not work[/li][li]worst case: destruction of the USB port[/li][/ul]
The only way to get around this is to feed additional power by means of a separate power supply unit.
So, any external optical drive without additional power supply is violating the USB specs. You can not expect reliable operation from such a device.