Why iMac eject DVD disks

Another help source (if you haven't already done so).

Go to your OS Help Menu. In the search field type dvd

Click on all the troubleshooting topics & Support Articles that pertain to your issue.

You can also do the same in Disk Utility. Open same up.

At the bottom left of the window, click on the purple button w/the "?" in the middle.

This will bring up the Help Menu.

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Try a different brand. Top forum favorites

DVDs

Maxell

Verbatim

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Apple's Disc Burning Quick Assist

Make sure the DVDs are not dirty, smudged and/or scratched.

http://support.apple.com/kb/HT2801Apple Computers: Troubleshooting the slot-loading SuperDrive

*** Your drive may need cleaning. Cleaning kits can be purchased from any store that sells CD/DVDs. ***

SUGGESTION FOR INTERNAL SUPERDRIVE

This might help. If you haven't done so yet, try installing Apple's SuperDrive Firmware Update 3.0, which is on Apple's support site at http://support.apple.com/en-gb/HT201953 to see if that resolves it. Although it's not specifically for that problem, the firmware update seems to be important for making external SuperDrives work in Yosemite, and it is one step I used to get my SuperDrive to work in a USB external case when plugged into my MacBook Pro.

Please let us know either way, whether the firmware update helps or not in your situation.