Forum:Loss of character help

I recently got the General Knoxx DLC and went through playthrough1 with my level 50 Lilith. After completing the playthrough i copied the savedata to make a second Lilith character to raid the armory on the second playthrough and save my main file to get to lvl 61 before doing the second. Howeve, after playing a LAN game with some friends while using my main Lilith file, the copy reverted to the point that i copied it at.

I am no backing up all my save data each time i close Borderlands and it is eating memory on my computer (i am playing on a Mac with a partion for Win7 to use steam for BL and TF2.) Does anyone know what caused this glitch and how I can avoid it in the future?