Using iMac in Target Display Mode with a MacBook Pro

I'd like to use my old iMac (27-inch, Late 2012) as an external display for my old MacBook Pro (13-inch, Mid 2009).

The hard drive in the iMac is failing, so until I get it replaced, I've installed MacOS Mojave to a thumb drive. I'm able to boot the iMac, no problem.

However, when I connect the iMac to the MacBook Pro and press Command-F2, nothing happens.

I know the iMac supports Target Display Mode, because I used to use it for that when connecting to my old work laptop.

It's the same cable I used to use, so it's not a cable problem.

I've successfully driven my LG monitor using the Mini Display Port on the MacBook Pro, so I'm pretty sure the port is fully operational.

What could be stopping it from working?

Is there anything about booting Mojave from an external flash drive that might stop Target Display Mode from working?

Or is it possible that the MacBook Pro just can't handle driving that particular display?

Any suggestions on how to get this working?

I don’t know if it’ll work with the Mini DisplayPort. I was stuck in a nasty half boot to grey screen problem last November on my 2011 27” iMac. While I was troubleshooting I tried the mini display port and tried TDM but it didn’t work. So I bought a Thunderbolt cable and then a FireWire cable or vice versa. The Thunderbolt cable worked... but I think the FireWire worked too. Now I’m starting to doubt my memory. I was going the other direction though. iMac TDM on a MBP. I tried the MDP port w/my old 2009 MBP & the 2012 MBP. Neither worked with my iMac. It That was just my experience. It’s worth trying a TB or a FW cable. I just remember the mini display port didn’t work for TDM for me. Which sucked because I had multiple mini display cables/adapters lying around from porting my laptops to the TV but I had a tough time finding the other cables in my town. The only Thunderbolt cable I could buy was super short. That’s how I’m certain that it definitely worked. I had to stack books to get the laptop close enough. I think I had no choice but to order the FireWire online. Hope you get it to work. :-)