Hi there
Yes, even using a 64 bit OS, you create the RAID array in the cards bios, before getting to the OS, and it will not let you select drives that create more than 2 TB. Adaptecs 'workaround' is to create a number of 'hardware' arrays, then stripe them together in the OS. This does work, however it is very wasteful if you are using RAID 5 as you drop one drive per array. In the end I use the card with one array per drive, then software RAID 5 them via Windows Server.
This works fine, and I get about 50 Mb/sec write speeds consistently, which is fine for my purpose. Again though the Adaptec card really does not like the Samsung drives, and will randomly 'drop' them on reboot. My other drives are Seagate, not WD as I had previously thought, but they detect every time without fail. Again this is not the end of the world, but it is a pain. Have to remember to set 'auto updates' to never, and remember never to reboot, but to shutdown, then restart. Apart from this the Adaptec card rocks!! you wanna buy it?