because the archlinuxm pogoplug does not have the same encryption keys as the stock pogoplug. the ssh client protects you against a possible forgery (someone trying to steal your passwords by having you connect to a server he controls)
anyway from your client computer go into the hidden .ssh folder in your user home directory (~/.ssh/ ) rename the known_hosts file as known_hosts.bak and retry. Most likely you'll get a warning about an unknown server and whether its keys should be stored. accept and login.
if you get a different error message please let us know.