Prohibitory sign 3 times at start, then it boot and works well

Hello Dan, thanks for the answer.

My iMac already have 2 disks, a SSD with the OS X system and this HDD, so that's why I doubt if the two problems are related.

As far as I saw in the Apple website the prohibitory sign it's about the system cannot locate the OS X boot properly. But my SSD is ok (i think...), the overheating problem is on the secondary HDD with has only files that I already backed up, but I still need to make a time machine backup of the SSD and copy it somewhere else.

Apart from that, after the prohibitory sign appears the third time, my system boots as normal, so if you want me to run some kind of test, I could do it from the same system.

You may have two drives (HD & SSD) but it appears the drive selected as the boot drive is the HD. Again you'll still need to get the system to boot up with a working OS to address things. Press the Option key so you gain access to the disk manager and try selecting the SSD to see if it will boot up. If not it does not have the needed files or it too is having issues.

Thanks for answering so quickly again.

I think you didn't understand me. My OS X system boot up perfectly after showing that sign three times. Is completely usable and I was using it (and I'm still) until now without any problem except that sign on the boot (which appears three times every time I turn on my computer or restart it).

I don't really know how the boot options of OS X works, but the secondary disk (the HDD) never had an OS X system installed on it. Also, from System Preferences > Startup Disk, I can only see my SSD.

Go into System Prefs and look at what Startup Disk reports as the startup drive. Select your SSD and reboot did that fix it? Basically, the HD is selected (even if it doesn't have a working OS) so it try booting from it then cascades over to the next found drive and then it finds it has a working OS to boot up from.

Hello Dan. Thank you again for the answer. I can't believe this, but it seems that in some moment (maybe when I upgraded to El Capitán) my system stopped defining the SSD as the default boot device.

When I went to see that preference before telling you about that the SSD was the only device there, I didn't noticed it, but if you have a default boot drive, there is a text below the device list saying it. In that moment, there wasn't any text in that window, now after selecting again the SSD as boot device, there is.

I reboot my system and guess what, no prohibitory sign anymore.

The question about the HDD overheating (and the S.M.A.R.T. error about it, that I don't know if could be caused by a bad sensor or something) is still around there, but you solved a great part of my problem, thank you.