I had a weird problem with my Palm Tungsten T5. When I had plugged it into my Windows computer, only the INTERNAL drive would show up in the file explorer. The external SD card wouldn't be accessible. The KB entry 8500 from Palm support lead to the solution: The system was trying to give the card the drive letter H but that was already occupied by a network drive.
After giving the external card a different drive letter in the drive manager, everything worked as it should.