If you don't have a CD-ROM attached on your amiga 1200, then you have to go to the sys:devs\dosdrivers and move the CD0 or Emergency disk icon to sys:storage\dosdrivers.
When you have a CD-ROM attached to Amiga 1200, then you need to open that file in a text editor and change the the device to your corresponding device, For example you have an Blizzard 1230 IV with SCSI kit and a CD-ROM. The device needs to chang to 1230SCSI.device. With SCSI CD-ROMs you have to know the Unit number of your CD-ROM.