If you unplug the RSP and start SDRT with only the R820T, does in still say it is unavailable?
Have you tried a different usb port? If not try that, I have a usb port on my desktop that refuses to talk to a R820T even though it loads, when all the other usb ports work fine.
Is it plugged into a usb hub? or straight to computer. Hubs can cause issues if not a powered hub.