Cubase enables internal microphone-- Why?

I thought I had it cured but I do not. Why does Cubase enable the internal microphone. I’m set at
Preferences / Sound / Input / Line-in ( Audio Line-in port ). So why does my mic get knocked out and only
internal microphone ( built-in ) take over. I’ve used different mic’s. Tested my 2 other computers and do not
have this problem with my older Cubase’s and they are both set “Line-in” also. What ever it is, it is something in
Cubase 6 that does this. Does anyone have an idea as to what can cause this? I also tried the audio “lines-in” with
the Garage Band that came with this iMac. No problem, therefore, it must be something in Cubase 6 causing this.
I can’t record any vocals. What can I do???

Well, I finally got the mic to work without activating the internal mic. Had to go to “Device Set-Up” and change the ASIO setting and it is now working great. Seems I’m learning more each day. Hopefully, I won’t have to bother you guys quite so much in the future. I do know you guy’s are always there when I need you.