Sony DSLR-A850 - Manual (Page 128)

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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.
MassStrg.card
Both cards 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.
Selected card Displays only the card selected with [Memory card].