What is mode is the USB connection of the camera set to? In order for the OS (and hence PM) to see the camera as removable disk, it should be set to M (Mass Storage), not P (PTP). In the latter mode, the OS sees the camera as Camera, not as drive. This has its uses (e.g., remote controlling the camera), but it limits some other functions (e.g., accessing the memory card).
You'll find the USB setting somewhere in the settings menu (don't have a D300 myself so can't tell you exactly where).
Hope this helps,
Hayo