Not if you are using OS-X. You will have to use Carbon Copy Cloner to make a bootable backup copy of your main drive onto your slave drive. After reformatting the main drive, you should use CCC to copy everything back to it from the slave drive. If you try to drag and drop a copy of OS-X, you will miss some invisible files and not be able to boot from that drive at all unless you do a system re-installation. If you are using OS-9, then a simple drag and drop copy will do the job for the OS folder and the files you want to keep.