My Imac won't send audio to my usb sound card

I had an Edirol UA 1EX sound card connected to my Imac but I went away for a couple of weeks and took my sound system with me. Upon my return I can't get the Imac to recognize the sound card. It shows up in System Preferences and the window opens ok, but if I click Default and the click Apply, it tells me that the card is being used by another application, close that application and the settings will apply. I've restarted and there are no applications open, still the same message.
What is it trying to tell me?

Well of course, it was trying to tell me I'm an idiot!
Though I'd selected it in Other category in System Preferences, I hadn't been to Hardware/Sound and selected the Edirol there, (slaps forehead). It's a while since I set it up originally.
Sorry for that.