Why doesn’t MacBook Pro 2009 to iMac mid 2011not work in Target Display Mode?

This is the hardware and connections that support TDM.

Use your iMac as a display with Target Display Mode - Apple Support

I have a MacBook Pro 2009 & a iMac mid-2011 and want to use the iMac in Target disk mode.

Actually, you said both ... and I assumed that you were referring to Target Disk Mode and answered as such.

The MacBook connector is actually a Mini DisplayPort, will this work or not?

Sorry, but this won't work. Target Disk Mode would require that each Mac would have to have one of the following connector types:

1. Thunderbolt 3 or Thunderbolt 2
2. USB-C (only available on 2015+ MacBooks)
3. Firewire

How to use target disk mode to move files to another computer - Apple Support

The 2011 iMac can only be used in target display mode with

another Mac that has Thunderbolt. There is no other way

around it.

BTW

Bill Davis wrote:

I have a MacBook Pro 2009 & a iMac mid-2011 and want to use the iMac in Target disk mode.....

You did say Target Disk in your post.