If you have a blue rings with standard software then the maximum filesystem size is 2Tb.
I have a blue rings with 2x1.5Tb - it's impossible to have a 3Tb partition (I've hacked mine as 2x1Tb unmirrored and 1x500Gb mirrored).
I'm guessing that if you put lenny/debian on it it might allow you to have 1x6Gb chunk - but I don't know.
There's two requirements;
1. Partitions > 2Tb
Needs GPT not MBR disks, blue rings come as MBR, I've not tried GPT, I suspect they *do* work (although you'd have to parted it on another machine, and muck around with the boot)
2. Logical partitions > 2Tb
Not supported by the blue rings kernel, so 2x2Tb mirror = 2Tb = OK 2x2Tb span = 4Tb = FAIL