Back Up Files in Mountain Lion with the Manual Method
If you’re too cheap to buy a second hard drive, the most rudimentary way to back up your files in OS X Mountain Lion is to do it manually. You would accomplish this by dragging said files a few at a time to another volume — a CD-R, CD-RW, DVD-R, or DVD-RW.
(If you use an optical disc, don’t forget to actually burn the disc; merely dragging those files onto the optical-disc icon won’t do the trick.) By doing this, you’re making a copy of each file that you want to protect.
Yuck! If doing a manual backup sounds pretty awful — it is. This method can take a long, long time; you can’t really tell whether you’ve copied every file that needs to be backed up; and you can’t really copy only the files that have been modified since your last backup. Almost nobody in his right mind sticks with this method for long.
Of course, if you’re careful to save files only in your Documents folder, you can probably get away with backing up only that.
Or if you save files in other folders within your Home folder or have any files in your Movies, Music, Pictures, or Sites folders (which often contain files you didn’t specifically save in those folders, such as your iPhoto photos and iTunes songs), you should probably consider backing up your entire Home folder.