When you backup your PSTs to a CD, you need to copy the PST to your hard drive and remove the Read-only flag before you use open it in Outlook.

When copying a PST to CD, always copy it back to your drive and verify it works in Outlook before deleting the existing PST.

If you use a CDRW, you can read directly from the CD.

To remove the read only flag, locate the PST using Windows Explorer. Right click on the PST and choose properties then uncheck the Read only attribute.