Raf_MegaByte wrote:
This morning I was nearby MacMini when I heard by chance a little beep sound and I noticed the power led was on.

I powered monitor to check what's happening, but it has no signal.

So I phisically powered on MacMini (led still on) and it made a long continue alert beep sound of 3 or 4 secomds, but then booted perfectly.

Powering off MacMini results in power-led still always on.

I must disconnet plug to stay safe.

I must also advice you I usually keep MacMini turned OFF but always plug connected behind a safety-surge multiple plug. So I should decline hypotesis it received a power over-tension surge from energy provider power cables.

Searching on internet returns no clues for the long continuous alert sound, I never heard as common Mac error signal, but also searching regarding power-on led always turned on seems to be known problem and best answer suggests that rear power switch of MacMini (that is electronic and not mechanical) could be damaged being elder...

Any time I connect again plug, I can hear Superdrive moving as to expelling a CD (No CD inserted) and power led automatically powering on (computer is off).

When I phisically power on MacMini two times it made that obnoxious loud alert sound any time I press rear switch-on button. Third time it made no alert sound (and crossing fingers it seems it doesn't made it anymore).

Also when shutting-down MorphOS, MacMini at power-off attempts ejecting CD (I can hear rumor from SuperDrive mechanics.

Any suggestions?

As for example:

Broken SuperDrive in short-circuit...

Broken switch on-off button on the rear of the Mac (as suggested on internet by an user) and requires sending MacMini to assistance.

MacMini automatic wake-up settings alive in the Open/Firmware or PRAM?

Does it require Reset PRAM, or modifing parameters via MacOS X, or entering OF?

Raf_MegaByte wrote:
SOLVED!

I faced rear of MacMini and there wes connected Firewire cable from External Hard Disk Enclosure I bought last month.

I unplugged Enclosure and I connected it to my Intel machine via USB, to check an old Hard Disk, but I left Firewire cable hanging void to the rear of MacMini.

Unplugging it made power-on led correctly shutting down.

It seems that keeping plugging Firewire cable makes MacMini waiting for a device that wakes it up automatically.

boot_wb wrote:
\o/

Congratulations. :)

Firewire delivers more power (7/8W) than USB (a mere 2.5W) so if it's supplying power to a common rail, that would easily be enough to power the superdrive and LED.
If it's just a case of power backwashing, rather than some weird 'listening' state (which seems unlikely if the superdrive remains powered), then a firewire cable modified to decouple the power lines would probably work if you wanted to leave the drive connected - that's the fix I used for my similar unintentional-power-via-USB problem.