I'm servicing a Mac whose user forgot both the user account password and the firmware password. I don't think he even remembers setting a firmware password in the first place, but I read somewhere that according to the system he was using, it is possible he might have done it while messing with the "remote lock" iCloud feature, but regardless, the point is the computer boots but can only be used in Guest mode.
Since the firmware lock prevents me from booting from a USB to reinstall the system and remove the user password, I thought I could get around this by removing the hard drive, putting it on a different iMac, doing a fresh installation and then putting it back into my friends computer once I know it's booting properly. Well, I tried that, but even though the hard drive works fine on a different Mac, when I put it back on his I just get the question mark icon.
It was booting fine before, so I guess what I'm trying to figure out here is if this is supposed to be a security feature, like, perhaps the firmware is locked to boot only from a certain ID in the partition or something, and now that that partition is gone it needs the firmware password so it can acknowledge the new ID and boot from it, or perhaps it is an entirely different problem.