diff --git a/rotate_bvecs b/rotate_bvecs
index 2e7dec8e30ba7a9fad3d18b2e514c147f8f6f155..e6aade98db552b3597867dee7fe3de0b4aa8b725 100755
--- a/rotate_bvecs
+++ b/rotate_bvecs
@@ -10,6 +10,15 @@ fi
 
 ecclog=$1
 bvecs=$2
+
+if [ -f ${bvecs}_old ];then
+    echo ""
+    echo "Error: Found that ${bvecs}_old already exists"
+    echo "This probably means that you have run this script before."
+    echo "If you want to force this program to run, please remove the file ${bvecs}_old"
+    echo ""
+    exit 1
+done
 /bin/cp ${bvecs} ${bvecs}_old
 #/bin/rm -f ${bvecs}