I read online that there was a command line to activate the partition option on Disk Utility:
diskutil partitionDisk disk4 1 GPT HFS+ newdisk
Which is just supposed to unlock the possibility to partition THEN chose the partition size or erase. Instead it renamed the disk but it also made all my data disappear!!
Please tell me there's a way to retrieve it or I'm going to smash the Mac to pieces...