Currently Being ModeratedSep 24, 2012 5:02 AM (in response to loredana.stan)
Open the disk to where you want the folder and select New Folder from the file menu of the Finder.
What format is the external "memory?" If it is NTFS, OS X can't write to it natively. There are some third-party programs that will allow you to write to it, Tuxera, NTFS-3G, and several others. Search google for OS X write NTFS or search MacUpdate.