The problem is not Linux and the disksize, as the kernel on your box supports large volumes. The problem is the partition table. The 'classic' MBR partition table (which is around since DOS 2.0, 1982) stores the start- and endsector of partitions in a 32 bit field, which means the highest addressable sectornumber is 2^32 - 1, ~ 4000000000. As a sector is 512 bytes, this means no partitions can be made beyond the 2TiB border.
Disks bigger than that have to use another partitioning system, for instance GPT
. (Actually I have never heard of another partitioning system supporting >2TiB).
The problem is that the 2Big v1 firmware doesn't support GPT partitions. And even if you would succeed in putting OS 3 on the box, it wouldn't boot, because the bootloader (some low-level software in rom) needs to load the Linux kernel from disk, which is located in partition 6. But the bootloader doesn't understand GPT, so it wouldn't be able to find the kernel.
However, as always there is a work around, only not using Lacie firmware. If you install a custom firmware, for instance fvdw_sl, and are not afraid for some low-level hacking, it is possible to generate a hybrid partition table. I wrote about that here
Unfortunately I can't install NAS OS 2 or 3.. The v1 is too old for that - That's what LaCie support claims anyway.
Basically Lacie just dropped support for the v1 models. They don't make firmware upgrades anymore. 'Too old' is nonsense.