Are USB 2.0 and 3.0 fully compatible?

USB is fully backwards compatible, and will give you the speed and power of the lowest version used.

BUT there is a few things you should know.

First is that a USB 3.0 port can provide up to 900mA, where as a USB 2.0 high speed port (most usb 2.0 within the last like 5 years or so are usb 2.0 high speed ports, the normal usb2.0 didnt last long) can only provide 500mA.

If you have a USB 3.0 hard drive or dvd drive, it can "work" in a usb 2.0 port, but you will likely be under powering the drive (which can cause damage that cant be repaired). So if you want to use usb3.0 hdd and dvd drives in a 2.0 port you either need an external power supply source for it, OR one of those usb 2.0 Y cables which give 2 usb2.0 ports worth of power to one device.

Some USB 2.0 devices such as mice and keyboards may not work properly in a usb 3.0 port, or may have limited functionality. Its device specific, so you would have to read up on the device to see whether it wont work with usb 3.0 or not.

As for your question, and on usb hubs in general.

When you use a hub, you are splitting the power and bandwidth between multiple devices.

This could cause problems if devices cant get the power they need or if you are hitting the bandwidth threshold, you may notice a lot of latency on connected devices.

If you buy a usb 3.0 hub, and use it in a 2.0 port, remember you are splitting a single usb 2.0 port worth of power and bandwidth, meaning you likely will not be able to populate all the usb slots on the hub, especially if any of the devices have a high power draw.

If you have a usb 3.0 hub in a usb 3.0 port, you can effectively make use of a hub when using multiple usb 2.0 devices (as usb 3.0 provides almost 2x the power, and over 2x the bandwidth), so it can handle multiple usb 2.0 devices without much trouble.

They also make hubs which solve the power problem by making them with a DC input to get external power from an AC adapter (it may or may not come with the adapter, but atleast has the option for it). And if you can get that, you can power as many devices as you want, but may still run into bandwidth issues if using a lot of high bandwidth devices (say a network adapter when you are doing a lot of downloading or uploading at the time, or when transferring to/from a usb hdd, it will eat most of the bandwidth of the usb port, meaning if you tried playing a game with a mouse/kb hooked to it when you were doing that, you may notice latency on button presses)

Lastly with hubs it generally causes problems for a few things. One being usb sound cards or usb headsets. There is often a ton of noise pollution getting through and you will hear hissing, buzzing, etc.

Also mice/keyboards which have LEDs can sometimes cause weird issues (atleast i know my corsair K95 with adjustable light levels causes problems when the light is on. The hub makes an audible tone).

So you want to avoid using hubs if possible. If not, its best to either get a usb 3.0 hub and put it in a usb 3.0 port (to eliminate power and bandwidth issues when using multi devices) OR get a usb hub that is externally powered is the next best option.

I dont recommend using a hub on a usb2.0 port w/o external power.

EDIT: people above dont know anything...

there are no usb 3.0 mice or keyboards.... i know for damn sure the corsair m65 isnt 3.0 as i actually have that mouse....

They are supposed to be backwards compatible, but it doesn't always work that way. A 2 mouse will have no issues plugging into a 3 USB port. But a newer 3 mouse might have power issues with a 2 USB port.

I have a USB 3.0 mouse and it doesn't get enough power from the USB 2.0 socket. So I would say not everything is compatible.

Its a Corsair M65 mouse

Edit: @filenotfound : http://www.pcmag.com/article2/0,2817,2416562,00.as...

5th paragraph then first sentence.

"The M65 is a wired mouse that connects via a tangle-free braided USB 3.0 chord. It doesn't offer a wireless option."