I solved it! If you don't use a DS3 Controller (Sixaxis and Dualshock 3 label, it must be with both), it looks like that PS3 doesn't let you update via USB your firmware, i tried flashing another CECH2004A without using a DS3 Controller and it doesn't work, instead using a DS3 it works perfectly. It seems that when you try to flash your NOR by recovery mode or by "Update via USB" in XMB mode, PS3 checks what kind of DS3 you are using and gives errors if it's not a DS3.
I flashed my PS3 and one of my friends' PS3 using a DS3 and it works perfectly, if you try with a different Controller it doesn't work.
Originally posted by another user
Make sure the usb is formatted to FAT32. And also on the usb the folders should be PS3 > UPDATE > PS3UPDAT.PUP.
It was a FAT32, file system was right!
Originally posted by another user
Which firmware you trying to install?
It wasn't about the firmware, i succesfully installed CFW, but the DS3 problem really pissed me off