When does macbook second HD work? I changed my optical drive with my old HDD and it works great. Now i have a SSD samsung 840 evo 250 GB. The battery doesn't last as long as before, but still works quite well.

The only thing it pisses me off is that sometimes the second hd starts working even if i'm not using it, does anyone know why? can i stop it?

I've try to google it but i couldn't find any solution.
Every time an application access a files on the HD, it will start itself. Sometimes it can be a background process who does this, that's why it may happen although you don't access a file directly. 

You can watch this with this command : lsof | grep "/Volumes/HDName"

lsof lists the files accessed by the all process. The grep selects only the files located on the HD. 

If you want to stop the hard-drive, just unmount-it.