FATFS copy between Ramdisk(FMC) and SD(SPI) on STM32F4
I understand that it's difficult to understand a problem from pieces of code, Just was wondering if someone has a similar problem. After a lot of tests and debug, I Probably understood the problem... 
Originally I put the two file systems FATFS into an array. 

Since I have two drives I declared:
FATFS FileSystems[FF_VOLUMES];

Instead, I tried to declare two different structures like:

FATFS FileSystemSD;

FATFS FileSystemRAM;

Everything worked perfectly. The copy between the two volumes is OK now. 
So If I use those structures inside an array, somehow, somewhere, data are messed up if we access to files on different volumes at once.
I checked the indexes in the code but I didn't notice anything wrong.

So, for now, I can't figure out what is going on, I will do more tests but this solved my problem.