Yeah ... I have done it to My N73...I copied all the messages that were on the phone memory into the memory card, and as i did that I noticed that the opening of messages whether to write or to read (compose, inbox, sent...etc) has become a bit slower...Frankly, I regret copying them into phone memory...I mean it isn't that big deal to be afraid to burden your phone memory, but i tried for days to copy the messages back to phone but in vain, or i couldn'y figure out how to do it...so i appreciate any ideas...
but for a girlfriend...consider it this way, you can hide your messages just by changing the sms memory choice and all your messages are hidden...