It seems to be a rough electric issue.
I tried 4 power supply, the same used for cellphones, all of them 5V and 1.5 A. It not seems to be differences between them.
1 on 4 works fine, the hd is working, but i didn't understood why.
However, i think i've asked something out of the scope of this forum...
I was curious about other member's experience about external hd's power supply.
Thanks a lot !