First of all, Poseidon 3.3 is absurdly old. I don't remember my version history, but it may even be too old to automount devices. Grab a new version as soon as possible.
Until then, open up Trident and look around. Has the massstorage class been invoked? If not, the device is incompatible and probably needs some custom software that will never be written for the Amiga.
Newer Poseidon is your best bet.