The TV must be one that will work as a computer monitor on HDTV and set to the port you're using before you boot the Mini... You should see a "no signal found" message from the TV before boot, which should change to the gray screen as the boot progresses. You might need to extend the sleep time on that TV so it doesn't sleep while the mini is booting. IF it does sleep (the screen goes blank or to a screen saver) you must be able to immediately wake it up - generally by inputting something from your remote.
The Mini upon boot should then automagically detect the TV.
OR you could connect two monitors, one not your HDTV so that you can boot to the nonHDTV then add the HDTV via "detect displays button" and enable/adjust the image via SystemPreferences:Display. especially if the signal or image from the mini looks bad.
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