Have u hacked ur N73 with Secman or HelloX carbide?? If yes ,then this is the reason why u cannot connect ir as mass storage.
To correct this the only way to coonect it is remove the memory card and Format the Phone Memory by *#7370#.
It will work after this.
If u haven't hacked it ,then try changing ur message storage to ur phone memory instead of memory card.
I had this problem too with my N73.