External HD name changed

My iMac has an external 500G HD (USB), and a 250G partitions is used for TM backup. The name of the partition was "Mac backup". Everything seemed to work fine until suddenly the name of the partition was changed to a stretch of letters and numbers. Why that happened? How to fix it? simply change the name back?
I think this happened after I turned TM off while the TM was backing up. But I am not sure.