3TB drives show up as 746.5GB !!

I've just bought three brand new 3TB drives which I've inserted into my JBOD ("just a bunch of disks") 4-bay removable Firewire 800 enclosure (Proavio S4) and for some reason they show up with only 746.5GB after formatting them! I've tried reformatting and partitioning to no avail.

The 3 drives are 2x Western Digital WD30EZRX and 1x Seagate Barracuda ST3000DM001. They all show the same drive sizes so it's reasonable to assume the drives aren't at fault, so I'm wondering if the drive enclosure has some issues. Unfortunately I have no way of testing out the drives with anything else at the moment. I've been using Western Digital 2TB drives (WD20EARS) without any problems in the same enclosure.

Am I missing something or is this likely a problem with the enclosure?

Ouch
You're right -I contacted Proavio and they told me the USB/Firewire bridge boards inside the enclosure don't support SATA 6G drives and there's no firmware update to fix it as it's a hardware limitation. Money out the window in other words!

If I had the cash I'd get an OWC Mercury rack pro which, unlike my rack, also has eSATA and USB 3.0 connectors on the back. And the drives don't need caddies to be installed. But that's US$ 680.
A cheaper solution would be to get a Newertech Voyager drive dock for US$ 75 which, like the OWC Mercury Pro Rack allows bare drives to be plugged in. Unfortunately it doesn't take two 3.5" drives at once (I regularly make backups of my drives) although I see it takes either 2.5" or 3.5" drives. I could buy two of those docks but that would mean more clutter and of course cost more.... any other (low cost) solution I could look into?