I have a MacBook Pro with Mini DisplayPort and I want to use target display mode to use a Thunderbolt iMac as an external display. If this works, would I need a Mini DisplayPort cable or a Thunderbolt cable?
According to this source, you won't be able to do that. You have to have a Thunderbolt Mac and use a Thunderbolt cable to use a Thunderbolt iMac as an external display. This other source has a table that confirms it. Sorry!