Now that the 400GB drive is inside the enclosure, how are you connecting to the iMac?
USB or Firewire? Firewire is the connection of choice.
You do not need to erase the drive, just initialize it. Kinda the same but different.
The drive inside the enclosure MUST be set to Master.

Where did you get the enclosure and is the Firewire bridge capable of HD sizes above 120GB?

P