Hi ! I finally managed to configure a RAID 1 with arcconf command (found on Intel RAID card LiveCD). I may come back on this thread soon, as I have some problem with additional disks : hot insertion...
I've been given a T5240 with 4 disks and 2 HBA cards (but no array connected). I did a factory reset on SP and NVRAM clean on OBP because the server had been used before.