Why doesn't my Superdrive mount DVDs?

Question

My late 2009 iMac's internal DVD started to ignore DVD's with content a few months ago. Insert a DVD and it sounds like it is trying to read it but then ejects it with no message. It still mounts CDs and blank DVDs but not DVDs with content. I can burn DVDs using Toast. I have a second external USB DVD recorder that reads the DVDs fine so it is probably not a system OS issue. Anyone have a clue?
Model Name:
iMac
  Model Identifier:
iMac11,1
  Processor Name:
Intel Core i7
  Processor Speed:
2.8 GHz
  Number of Processors:
1
  Total Number of Cores:
4
  L2 Cache (per Core):
256 KB
  L3 Cache:
8 MB
  Memory:
16 GB
  Processor Interconnect Speed:
4.8 GT/s
  Boot ROM Version:
IM111.0034.B02
  SMC Version (system):
1.54f36
OPTIARC DVD RW AD-5680H:
  Firmware Revision:    3AHB
  Interconnect:    ATAPI
  Burn Support:    Yes (Apple Shipping Drive)
  Cache:    2048 KB
  Reads DVD:    Yes
  CD-Write:    -R, -RW
  DVD-Write:    -R, -R DL, -RW, +R, +R DL, +RW
  Write Strategies:    CD-TAO, CD-SAO, CD-Raw, DVD-DAO
  Media:    To show the available burn speeds, insert a disc and choose View > Refresh

Answer 1

Try resetting the SMC http://support.apple.com/kb/HT3964 and PRAM http://support.apple.com/kb/HT1379