Assuming $LFS is set to /media/LFS (you're a bit limited with the info you provide) this should read: /media/LFS/sources/gcc-build
You probably did the following step from within the wrong directory:
mkdir -v ../gcc-build
It should be done from the gcc directory (/media/LFS/sources/gcc-4.5.1) and not from the sources directory (/media/LFS/sources).
Hope this helps.