JAET, on 13 November 2012 - 01:24 PM, said:
My MacPro logged out after a session.
When I tried to re-start it stays on sppining wheel.
I tried rebooting from an external USB drive with MAC OS X 10.6.3 clone but it gives a padlock and an option for a password, but my password does not help.
At some point in the past someone enabled the firmware password protection on your machine - possibly before it was your machine if you're not the original owner - the effects of which include blocking unauthorized booting from external media.
Given that you presumably don't know that password, the way to clear it is to open the machine, change the amount of RAM and then turn the machine on while holding cmd-opt-P-R until the machine has chimed three times. After that you can shut down and put the memory back to the way it was.