How to view the files on mnt\sdcard on PC and how to set default download and save location
It seems that, like on the Atrix, /mnt/sdcard is your internal storage, while /mnt/ext_card is your real sdcard. You can verify this if you check their seizes: You know the size of your sdcard, so you should be able to point that out. The internal will probably have a size which is not a multiple of 8 (most likely, it will be 6 GB or the like).

If you are unsure how to check this, you could e.g. use SanDisk Memory Zone1 or, even better, Storage Analyser2, which should make it easy to point out: