Q: iMac (27", Mid 2010) not booting / forgot Firmware password

Hi everyone,

I'm using an iMac (27") from Mid 2010. Mac OS version is Yosemite.

Everything worked perfectly until about last week. I shut the system down in the evening and tried to start it again on the next day. After the chime, the apple logo appeared and soon after the progress bar. But it only goes until about 70 % and stops there. I let it load for more than an hour and it still didn't boot, so I think something is wrong. But I thought to myself: "No problem, just use your time machine backup to reset it to yesterdays state." BUT: When I try to enter the recovery mode, it's asking me for the firmware password which I don't know anymore. I tried some passwords I likely used back in the day when I set the password, but didn't get it right. Then I thought it would be a good opportunity to install a clean version of El Capitain, but without the password it won't even boot from another media than the built in hard drive.

So, it there a way to reset that password or to boot from another media (USB, DVD) without having to enter it?

Best regards,

Marco