According to Murphy’s law: “A transistor protected by a fast-acting fuse will protect the fuse by blowing first”.
That said it can’t hurt to have a fuse in your circuit. But there are a couple of other things I would have done.
1) Add a big electrolytic capacitor to support the motor with some extra current at startup. 2) Remember to add a diode on your motor as the back EMF if anything could kill your USB port.
I can’t see anything bad happening. As far as I can tell the limit of the USB port is 500mA so if you would hook up a motor that draws 1A, it would only run at half speed. (but be advised to use diode for things like motors or solenoids). So if you have an external disc on the same port (via a hub) and that draws 300mA, that would leave you with 200mA for your device. Why don’t you build yourself a current probe like this https://www.robotshop.com/letsmakerobots/node/23097 and measure the current?
I would always prefer using an external USB hub with overcurrent protection. Spend some more money for a good USB hub. I have killed a few of the cheaper USB hubs without any protection like this one:
At the moment I use this model and it works fine. It must have a hardware overcurrent protection inside. If a device draws too much current it will shut down and you will need to repower it.
