garageband midi lost

I have just a minor issue, however it's very annoying. I have a MIDI Keyboard (piano) that I plug into my iMac and operate through Garageband. However there are times when I need the USB cord connecting these two for something else, such as my printer, and when I disconnect the keyboard from the Mac Garageband pops up and says WARNING WARNING WE'VE LOST CONNECTION WITH A MIDI DEVICE (well not really like that, but it says there's no input available). This was well and good until it started messing with what I was doing, Whenever my keyboard would go to sleep or I would try to print a page I have to minimize garageband and set up the printing over again. Worst is when I'm typing something and don't realize the keyboard turned off and garageband pops up and blocks the last couple words I typed in.

Like I said, simple, dumb problem that, after a while, gets quite annoying. So, does anyone know how to disable the feature of Garageband warning me an input is missing? Thanks a bunch!