Using iMac (late 2009 27 inch model) as additional monitor for Apple Mac Pro (Late 2013)

Hi there,

I am trying to activate target display mode on my 2009 iMac to no avail. According to apple's instructions this should be possible, but when I connect the two computers with a thunderbolt cable and press command-F2 on the iMac I get an error sound and nothing happens. I have tried troubleshooting (turning off/on, resetting MSC, experimenting with different thunderbolt ports on the Mac Pro and reinstalling OS X) but to no avail.

Could you please help with any additional suggestions? The thunderbolt chord is functional (I have tested it) but is there a way to test whether the Minidisplay port is functional/ active?

Thank you!

A