I have a late 2010 Macbook Pro (MacbookPro6,3) running Lion (10.7.3) and an iMac of indeterminate vintage (iMac11,3) running Snow Leopard (10.6.8). I have a Thunderbolt cable. Neither mac supports Thunderbolt; both have MiniDisplayPort connectors. I want to run the iMac in Target Display Mode as a second display for the MacBook Pro. Can I make this happen? I know that the iMac supports Target Display Mode with a MiniDisplayPort <> MiniDisplayPort cable. The tech at my campus Apple store sold me a Thunderbolt cable (same connectors as MiniDisplayPort, but with active electronics, right?). Target Display Mode is not working -- I press cmd+F2 on the iMac and nothing happens. I can see several potential problems: 1) different OS versions interfering with target display mode. 2) thunderbolt cable is incapable of acting like a passive MiniDisplayPort cable.

Do I need to search out an old MiniDisplayPort cable to make it work, or is this doomed to fail because of the mismatched OSes?