AHT not functioning

I'm trying to run Apple Hardware Test on my iMac (27-inch, mid 2010). The SuperDrive failed a long time ago and I'm using a wired keyboard and an Apple external CD drive. I have the original CD containing AHT.

Starting up holding down the 'D' key shows a globe, followed by the usual 'support' overlay. It won't boot from the external CD drive or a memory stick.

Can anyone offer a solution, please? I can't believe that failure of the internal SuperDrive would preclude any use of AHT.