I've tried a Sweex usb 2.0 card reader and it gave me the
same trouble as you are experiencing.
I asked about it in the Poseidon yahoo group, but never
got an answer. Then I read on EAB that the cardreader
from Amigakit should work flawlessly with Poseidon.
I decided to take the risk and bought one. I never
experienced trouble with it ever since I've installed it.
Regards,
Lloyd