The first message is completely unrelated. When your computer boots, a program called IPrefs is run which sets the resolution and color depth (number of colours available) of the Workbench screen. To do this it must close the existing screen and open another - but it can't do this if you already have windows open on the screen.
As for the CD-ROM... I would have thought the unit number would be 0. You could try this. Open a Shell or use the Execute Command function from Workbench, and type:
ed devs:dosdrivers/cd0
You should find a line that says "Unit = 3", try changing that to 0 (zero). Save the file back out (you should be able to press [ESC], then x, then [ENTER]) and reboot.
I have no idea if that will work of course but no-one else has replied :-D
