I assume your in the preview program as backwards compatible has not been released to the public yet. you would be better off posting in the preview forums
forums.xbox.com/.../default.aspx it really is the best place for help and info with the preview program.
Try hard resetting your xbox (5-10 seconds holding the power button) and remove any usb HDD if you have one attached, plug it back in once the xbox has rebooted and see if that resolves your problem.
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