Sony Alpha DSLR-A850 Instruction Manual page 128

MassStrg.card
When "Memory Stick Duo" media and a CF card are both inserted in the
camera during mass storage connections, you can select the displayed
method on a PC.
Both cards
Selected card
128
Displays both the "Memory Stick Duo" media and CF card/
Microdrive. This is supported by a PC running Windows
2000 SP3, or a later version of OS.
Displays only the card selected with [Memory card].