Good morning, Chris. I'm a registered Poseidon 3 user who has recently updated to version 4, included in MorphOS 2.0. In my clean install of the operating system, Poseidon refuses to recognize the same USB stick that worked wonderfully in Poseidon 3 in MorphOS 1.4.5, and also in my work's PC.
Upon inserting it, nothing happens, the stick's led does not blink at all, and all I can see in Trident's debug output is this:

SET_ADDRESS failed: response timeout (6)
Device enumeration failed, sorry.

If I restart all classes, two pop up windows come up immediately, about the device being connected (and bound to "massstorage.class"), and disconnected, and perhaps not in that order. But no device is mounted.
The most intriguing thing is that I also have a memory card reader, which does work. There is no additional hub, aside the one provided by the Pegasos 1 (April 2) mainboard.
I've red somewhere that you are looking into this, and expect you can come up with the reason and solution to this problem. I hope you are not so tired about all this USB thing!