I had installed a 500GB drive into the top drive slot in the DNS-343 and as a test I copied over about 90GB of data. I then took a 750GB drive and put it in the top slot, and put the 500GB drive into the second slot. The DNS-343 prompted me to format the 750GB drive, said it succeeded, it then gave the option to restart the DNS-343, which I did.
After the DNS-343 started up, I logged in to the config menu, and it prompted me to FORMAT the 500GB drive that I had previously used in the 1st slot (but had moved to the second slot)
The DNS-343 should not require you to reformat a drive just because you moved it to the second slot from the first slot.
Both drives were prepped as EXT2.
Mike