Thanks for both of your replies.
Yes the 3TB is slightly slower, since its older, it gets around ~110MB/s.
I was expecting it to work since they say so in this page: http://blog.covecube.com/2013/05/stablebit-drivepool-2-0-0-256-beta-performance-ui/ they even show it working..( http://blog.covecube.com/wp-content/uploads/2013/05/read_duplicated.png )
I didn't really tested this before, since a single HDD would hit the 1Gb NIC max speed, but now that i got a 10Gb NIC, i was hoping to get a bit more performance out of this.
EDIT: Just tested, copying 3 different files, all of them are duplicated 2x. Drivepool seems to insists in using only the 4TB drive?! (see screenshot, i think i showed all relevant stuff)