How to view the files on mnt\sdcard on PC and how to set default download and save location The directory mnt\sdcard is the location where all the downloads and the received files are saved, however this file cannot be located on the PC if connected using USB Cable. Moreover, it seems that this file is on the Phone's Internal Memory and not in the Memory card
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: