Answer:
If you plug a USB 2.0 hard disk into a USB 3.0 port, the device will function normally and will be compatible with the USB 3.0 port. The USB 2.0 hard disk will be able to transfer data at the maximum speed of USB 2.0, which is 480 megabits per second (Mbps).
However, if you plug a USB 3.0 hard disk into a USB 2.0 port, the device may not function properly or may be limited to the maximum speed of USB 2.0.
In general, USB devices are backward compatible, which means that you can plug a USB 2.0 device into a USB 3.0 port or a USB 3.0 device into a USB 2.0 port. However, the device may not be able to take advantage of the full speed and capabilities of the newer port.