Why does my hard drive not spin up

Hello,

On my iMac 24" EMC 2134 2.8GHz I have experienced what I thought to be a hard drive problem:

When I powered up the iMac I had the white screen, then the Apple sound, then I heard the fans spinning up and some (normal) noise in the superdrive (that is empty). Then I heard the hard drive trying to spin up, then spinning down, making some ugly noise (like TAC TAC), then trying to spin up again, spinning down again, making again the TAC noise, endlessly.

Thinking that it was a hard drive problem I replaced it, reinstalled the system on the new hard drive and everything worked fine for 5 days.

Until this evening, when I tried to start my iMac and find myself with the same problem, but now with the new hard drive: tries to spin up, spins down, makes some noise ... endlessly, and the system does not boot.

I am asking myself the following questions:

Could it be the power supply that is failing? Why did the new hard drive work during 5 days?

If the problem comes from the power supply, why does the screen, the fans, the superdrive, the sound work?

After this, I have tried the old hard drive (the one that I thought to be faulty) in an external dock, and I realised that it spinned up properly and that I was able to boot from it when placed in the external dock.

I am asking please for help finding the cause of the failure.

Thank you

RP