Avrdude error when uploading sketches from Arduino UNO to ATmega328 stand alone

Hello, I have an Arduino UNO board and I was trying to use it to upload sketches in another micro ATMega328 with bootloader already in (I bought a micro with the Arduino UNO bootloader already installed).
I followed the instructions I've found here: BioPhysEngr Blog: Writing sketches to a ATmega328 pre-loaded with the Arduino Uno bootloader using an Arduino Uno and ArduinoISP, but when I get to the last point (click on "Upload using programmer") I get the following message:

avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override this check.

I read the troubleshooting session on this topic, BUT:

  • the board I've selected from the Tools > Board menu is Arduino UNO, so it appears to be correct;
  • the avrdude version should also be OK. I read that it could be a problem if you have an ATMega328P vs ATMega 328P-PU, but the version printed on both micros is ATmega328P-PU, so they match each other.

Bottom line, I cannot understand why I get that error when trying to upload sketches from my Arduino UNO to an ATMega328 micro stand alone.
Is there someone who can support me and maybe make some recommendations on what else to check ?

Thanks.
Regards.
Claudio

have you tried to change the port you use to connect the UNO??? i worked for me when this happened