Adding files to a CD

If you've written and closed the CD (which is presumably the default) you can't add to it. You can only erase the entire disk and start again. You need to have left the disk 'appendable' when you burned it. This is the whole process (it also works with non-rewriteable CDs) - it may be different in different OS versions:

 1. Put all the material for your first burn into a folder.

2. Open Disk Utility. Choose Images>New>Image from Folder. (May be File>New>Disk Image from folder.) Select the folder you want to burn.

3. Type a name for the disk image and specify a location. Click Save.

4. In Disk Utility, click the Burn icon on the toolbar. Navigate to, and click once, the disk image you just created.

5. A Burn Disc dialog box will open. Click the blue triangle at upper right (or the disclosure button). When the dialog box expands, click 'leave disc appendable'.

(It's greyed out because I haven't inserted a disc).

6. Click the Burn button to burn the CD.

This creates your first session. To add more material:

7. Repeat steps 1-5.

8. In the dialog box, the button at lower right now says 'Append'. Click this to burn the new material.

When you mount the disk, two disk icons will appear on your desktop. You can go on doing this until the disk is full.

Apparently the 'Burn' icon (and the facility) was helpfully removed from Disk Utility in El Capitan. In Step 4 you would now select the disk image in the Finder and right- or control click the image and choose 'Burn Disk Image (title)'. then proceed as I indicated.