I just restored my HD from a Time Machine backup and afterward my AHT won't work, despite the fact that I have the original install disk (10.5 Leopard, disk #2 that says it has AHT on it). AHT worked a week ago, no problem. I'm 100% sure I have the right disk. I've successfully run AHT many times using this disk in the past.
Weirdly, I can still boot to Recovery Mode from the same disk by hitting Command + R (unfortunately, can't reach AHT that way). I ran First Aid from disk just in case, no problems found. So the disk drive, disk, and keyboard all seem to work okay.
I shut down, hit D right after hitting the power button, and screen goes gray forever, never boots. Same thing occurs when pressing Option at boot to choose the boot disk... goes to gray screen, never boots. I'm using a wired USB keyboard (a Microsoft brand, but I'm accounting for default option key vs. command key vs. windows key, etc.). I also tried hitting D at different times after hitting power, before hitting power, and tried different USB ports for the keyboard. Unplugged everything. All fails.
I assume my HD recovery changed something. Apple support said they think my disk is bad, which I don't buy since it worked last week.
Perhaps relevant: I want to use AHT because — before I recovered my HD — AHT indicated I had bad ram. (Ram problems were intermittent). I wanted to determine which of my two chips was bad by using just one or the other and repeatedly running AHT to find which individual chip is bad. Currently using just one of my two 4GB chips, which I guess could be related, if it's the bad one. MacMini is a 3,1 Intel currently running 10.11.6 btw.
Thanks a lot for any advice.