Let me start by saying that I actually was going to return the two ProRaids I bought, and would have left a 1 star rating. But thanks to the help of some Mediasonic forum member, I was able to fix all.
I bought two of this things to setup in 2xJBOD mode each one, width 4x3TB and 4x2TB hard drives on each one, which gives me 12TB and 8TB on each ProRaid.
The first problem was actually cause by one of the four WD Red I just bought. SMART was telling me all four where ok, but after many tried to copy information to the ProRaid, the 1st 3TB hard drive on the line failed, and had to be replaced.
It was actually good cause it was much easier to ship the WD RED for replacement, than to ship this big ProRaids.
So, after changing the faulty hard drive, all was fine. I completed 4x3TB hard drives on the top "layer" of the ProRaid (a ProRaid can be used in full or in half, and each half they call it a layer) of each ProRaid, and all was fine.
But when I tried to add the 4x2TB hard drives to each ProRaid, one of them would get into error mode and kinda freeze, and the second one would only let me use 2 of the 4 hard drives.
To make this review short, this is how you should try to add the second layer if you didn't do it all together, and you are having problems.
1) Turn OFF the ProRaid
2) Remove the hard drives from the original layer (that where rightly recognized, in my case at layer 1)
3) Fill the non working layer with hard drives (actually, the hard drives where already there)
4) Turn the ProRaid ON.
5) Pressed the "Change Modes" for 3 seconds to start the raid initialization.
6) Hold down the confirm button on the back until ProRaid turns OFF.
7) Turn the ProRaid ON. At this point, all 4 hard drives should be recognized perfectly.
8) Turn ProRaid OFF
9) Inserted the original hard drives on the empty layer.
10) Turned ProRaid ON, and you should have all 8 hard drives recognized in 2 independent JBOD ARRAYS without loosing a single bit of the original array!
I used this same method for the second ProRaid I have, which, instead of only allowing me to use 2 of 4 hard drives on the 2nd layer, would actually give a hard drive error and go into "error mode".
The only difference is that steps 4, 5 and 6 have to be done more or less quickly, or the ProRaid would get innto "error/freeze mode".
NOTE: WHILE I DIDN'T LOST A SINGLE BIT OF INFORMATION FOLLOWING THIS METHOD, DO IT AT YOUR OWN RISK!
NOTE2: CREDIT HERE: http://forum.mediasonic.ca/viewtopic.php?f=27&t=1538&p=6944