What CFW are you running?
Do you have multiple usb devices plugged in?
If on Rogero, I would update to the latest FULL mM then update to the latest update. See here:
I recall reading that if you have multiple usb devices plugged into the PS3 that it would only recognize one of them. I can't remeber which version of mM or Rogero it was. Try updating mM and plugging one usb device to see if it will show up.