Let me start by saying that I was actually going to return the two ProRaids I bought, and would have left a 1 star rating, if not for some information on the Mediasonic forum.
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 caused by one of the four WD Reds I bought. Even though SMART was telling me all four Reds where ok, one of the 3TB Red hard drives was broken, and had to be replaced. So, check the hard drives.
So, after changing the faulty Red, all was fine. I added 4x3TB hard drives on the top "layer" of each ProRaid (a ProRaid can be used in full or in half, and each half they call it a layer), and all was fine.
But when I tried to add the 4x2TB hard drives to the second layer of each ProRaid, one of the ProRaids would get into error mode and kinda freeze, and the other one would only let me use 2 of the 4 hard drives.
To make this review short, this is how you should really add the second layer if you haven't added all the hard drives at the same time the first time and you are having problems.
1) Turn OFF the ProRaid
2) Remove the hard drives from the original layer (that where rightly initialized, in my case it was the top layer)
3) Fill the non working layer with hard drives
4) Turn the ProRaid ON.
5) Press 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) Insert the original hard drives on the original (empty) layer.
10) Turn the 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