I can tell you it mostly definitely should work if you select "Mass storage mode" when plugging in the USB cable. I have used mine on my PS3 without any problems at all, it sees both the internal storage and SD card. They show up in the menu as "USB Device (N900)", once for internal and once for SD, although you do have to press triangle and Display All.

It could perhaps be that the Samsung is confused at seeing two drives instead of one. Try with and without an SD card if you can, perhaps it will see that, although it does kind of defeat the purpose. I have noticed devices in the past that do not like it when two Mass Storage devices show up on a single USB port and refuse to see either instead of defaulting to the first found.

The beauty of the N900 is unlike older Nokia devices, it also will charge while you are using it in Mass Storage mode due to the combined charge/USB port.