bug: Target Display Mode + optical audio cable = black screen

Using Target Display Mode on my iMac 27" (late-2012) with my MacBook Air (mid-2013) always resulted in a black screen on the iMac.

Hardware used:

• MacBook Air mid-2013 (Mac OS X 10.8.4)
• iMac 27" late-2012 (Mac OS X 10.8.4)
• Thunderbolt cable between MBA and iMac
• optical audio cable in iMac (digital audio out through optical mini jack plug)

Now, enter Target Display Mode on the iMac keyboard (Cmd+F2).

Result: black screen on the iMac (MBA is okay, optical audio is okay)

Workaround: unplug optical audio cable before entering Target Display Mode. Plug in the optical digital audio out cable after TDM is working and all is fine.

I sent a bug report through the feedback form. Just posting here to save others the few hours I had to spend finding the cause and workaround!

-- Richard