My disk drive reads CDs, but it won't read DVDs anymore.

See "The drive accepts discs but they do not mount or are automatically ejected" > http://support.apple.com/kb/HT2801

If you are running 10.6.8, then you have Snow Leopard.

Cures for an uncooperative optical drive

At worst you will need to take it in for service. The drive could be bad, out of alignment, or have a dirty lens.

Reinstall OS X without erasing the drive

Do the following:

1. Repair the Hard Drive and Permissions

Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

2. Reinstall Snow Leopard

If the drive is OK then quit DU and return to the installer. Proceed with reinstalling OS X. Note that the Snow Leopard installer will not erase your drive or disturb your files. After installing a fresh copy of OS X the installer will move your Home folder, third-party applications, support items, and network preferences into the newly installed system.

Download and install the Combo Updater for the version you prefer from support.apple.com/downloads/.

You can't without an installer DVD. I don't know what is exactly wrong. All that does is reinstall OS X in case your problem is due to a corrupted system.

There is no Mountain Lion DVD. You purchase and download it via the App Store. You must be running 10.6.8 or later. Before you download see this:

Make Your Own Mountain/Lion Installer

1. After downloading Mountain/Lion you must first save the Install Mac OS X Mountain/

Lion application. After Mountain/Lion downloads DO NOT click on the Install button.

Go to your Applications folder and make a copy of the Mountain/Lion installer. Move

the copy into your Downloads folder. Now you can click on the Install button. You

must do this because the installer deletes itself automatically when it finishes

installing.

2. Get a USB flash drive that is at least 8 GBs. Prep this flash drive as follows:

a. Open Disk Utility in your Utilities folder.

b. After DU loads select your flash drive (this is the entry with the mfgr.'s ID and size) from the left

side list. Click on the Partition tab in the DU main window.

c. Under the Volume Scheme heading set the number of partitions from the drop down menu to one.

Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the

partition scheme to GUID then click on the OK button. Click on the Partition button and wait until

the process has completed.

d. Select the volume you just created (this is the sub-entry under the drive entry) from the left side

list. Click on the Erase tab in the DU main window.

e. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, check the

button for Zero Data and click on OK to return to the Erase window.

f. Click on the Erase button. The format process can take up to an hour depending upon the flash

drive size.

3. Locate the saved Mountain/Lion installer in your Downloads folder. CTRL- or RIGHT-click on the installer and select Show Package Contents from the contextual menu. Double-click on the Contents folder to open it. Double-click on the SharedSupport folder. In this folder you will see a disc image named InstallESD.dmg.

4. Plug in your freshly prepared USB flash drive. You are going to clone the content of the InstallESD.dmg disc image to the flash drive as follows:

a. Double-click on the InstallESD.dmg file to mount it on your Desktop.

b. Open Disk Utility.

c. Select the USB flash drive from the left side list.

d. Click on the Restore tab in the DU main window.

e. Select the USB flash drive volume from the left side list and drag it to the Destination entry field.

f. Drag the mounted disc icon from the Desktop into the Source entry field.

g. Double-check you got it right, then click on the Restore button.

When the clone is completed you have a fully bootable installer that you can use without having to re-download Mountain/Lion.

Note: The term Mountain/Lion used above means Lion or Mountain Lion.

As an alternative to the above (you still have to do your own download of Lion/Mountain Lion) you can try using Lion DiskMaker 2.0 that automates the process of Steps 2 through 4.

I don't know. Never tried it because I don't have a computer with an SD slot. If you know the computer will boot from an SD card, then it should work the same as on a flash drive.

Yes, if you repartition the device everything is deleted. You don't want to do that. You cannot use the device for the installer without first erasing it. Get a new flash drive if you cannot erase what you now have.

You can't fix the optical drive. But now you don't need an optical drive to install or reinstall OS X. But now you can reinstall Snow Leopard from the USB flash drive or use Disk Utility to repair the hard drive and/or permissions.