Has a Time Machine backup overwritten all external files?

Hi,

I just upgraded to snow leapord and ran a backup using Time Machine to my external hard drive.
Now rather than my external hard drive being named 'My book', it's called 'Time machine backup' and everything that was saved on the external hard drive to begin with is gone! Instead all I've got is a folder 'backups/latest/ etc.... I'm missing alllll my music, would be an absolute nightmare to upload all that stuff again. Not to mention there's other stuff that was on there I wanted to keep.
Has doing a backup deleted everything that was on it initially? It definately didn't warn me beforehand.

Any help much apprecaited.

Adam.