The CDs you use for backup fall broadly into two categories: CD-R and CD-RW.
CD-R are recordable. You can burn information onto them, provided you do it all at once. Once you have “burned” the CD you can never add further information. In New Zealand you can buy inexpensive blank CD-Rs for about $1 each.
CD-RW are rewritable. You can burn information today and then reuse them tomorrow. These CDs cost a little more. In New Zealand you may pay about $4 per CD.
There is a great deal of information about CDs at the CD-Recordable FAQ:
If you want to re-use a CD-RW in your Mac you must first erase it. According to Apple’s Knowledge Base article:
If you want to reuse a CD-RW disc, you must first erase the existing information on it. Before you erase the disk, make a copy of any files you don’t want to erase. Erasing, reformatting, or preparing a disk for burning permanently deletes all of the disk’s contents.
- Insert the disc.
- Open Disk Utility (Applications/Utilities)
- Click the Erase tab.
- Select the disc from the list of drives on the left. Make sure not to select the hard disk and make sure to select the device icon, not the volume icon or the Erase option will be dimmed.
- Click Erase.
CD-RW discs may deteriorate after being erased many times. Check the specifications for the disc to find out its expected life.