su username

sudo mount /dev/sdb1 /mnt

sudo command...

sudo -s

sudo -i

myusername:*:16992:0:99999:7:::

$ su
Password:
su: authentication failure
$ sudo su
# ls -l /bin/su
-rwxr-xr-x 1 root root 31092 Jun  5  2012 /bin/su
# chmod u+s /bin/su
# ls -l /bin/su
-rwsr-xr-x 1 root root 31092 Jun  5  2012 /bin/su

su - git
Password:
su: Authentication failure

sudo su - git