some devices actually have their sdcard in a folder named 'sdcard-ext'
I have a motorola droid bionic. Motorola tried to prevent issues with mounting and unmounting the external sd storage,so they partioned the internal storage and mounted part of it to a folder under /mnt/sdcard. to access the REAL sdcard I have to save files to /mnt/sdcard-ext.
I have all my fpse files (isos, mem cards, save states, bios) on the real sdcard, not the /mnt/sdcard. most devices aren't setup like mine though.
hope that answeres your question. once you know you may need to go to /mnt first it should be easy