P-O-S?
I had a generic card reader (Comotron) that had issues like what you describe. I recently bought a reader from ATech which has been flawless. No drivers required - just plug into Windows or Mac and go.
I've copied and deleted files from SD cards that are used in my Pentax Optio S and my Palm T3 - no problems. I've moved cards to and from the reader to the devices and back again - no problems.