Thank you friends for your advice.
Some information and my opinion now that I have all up and running again:
This HDD has just one year in a new machine. I've been using SME with this same configuration for years and I changed machines every 2 years more or less.
The problem was produced by an electrical problem during a storm. The operating system failed, not the rest of the data on disk.
I tryed to use the SME rescue solution booting from the installation CD, I could see that the data was there, but again I couldn't find a way to read the data inside the ruined the HDD and copy it to other disk. I think that I should have to mount an extra HDD too

The method of copying the data using a Windows machine was simple for me with not much experience with Linux and solved the problem, but I am not happy with that. I would like to know how to do this the old fashion way, using the native Linux tools.
I know that the RAID disk storage is much better, but I have some concerns about using what I don't understand very well. If fixing a RAID system after a crash would expose me to the same lack of information of that in mounting an extra HDD to an SME server, it definitely wouldn't be a solution for me either. Do you know if there is any good, basic information about how RAID works, how to install and ***specially*** how to fix a RAID system in case of a disaster ? If so, I would study it, spend some time on this and give it a try. If not, I can't imagine how could I read a RAID in a Windows PC

I have an extra HDD in my Windows PC to make backup from time to time, but in this case I needed information that was not included in the last backup. I would like to have and extra HDD to make automatic programmed backups directly inside the SME server.
Finally, I couldn't find information on how to easily mount an extra HDD to my SME server, and I still think that this should be something really simple for somebody with a little much more experience than mine. I also think that not everybody uses RAID in simple configurations.
Thank you all again, as always all opinions are very valuable to me.