I work in a small production company and have 4 x 17" Mac Book Pros. One is brand new and the other three are all a couple of years old, but they all have the most up to date software, drivers etc.
The brand new one and one of the old machines have the issue of the SxS card not mounting on the desktop, although you can see the card in the menu bar, so it's definitely recognising the card. It will also mount perfectly from a USB card reader.
It's just so odd that the issue occurs on two of the machines, but not the other two.
I'm rather hoping you can shed some light on the matter and look forward to hearing from you.