To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

ubuntu@ubuntu:~$ sudo fdisk -l

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000dd8c2

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1       42210   339051792   83  Linux
/dev/sda2           42211       54223    96494390+   f  W95 Ext'd (LBA)
/dev/sda3           54224       54236      102400    7  HPFS/NTFS
/dev/sda4           54236       60802    52736000    7  HPFS/NTFS
/dev/sda5           42211       46926    37881238+  83  Linux
/dev/sda6           47310       54223    55536672   83  Linux
/dev/sda7           46927       47309     3076416   82  Linux swap / Solaris

Partition table entries are not in disk order
ubuntu@ubuntu:~$ sudo chroot /media/disk/
root@ubuntu:/# grub-install /dev/sda
/usr/lib/grub/grub-mkconfig_lib: 38: cannot create /dev/null: Permission denied
/usr/sbin/grub-probe: error: cannot find a device for /boot/grub (is /dev mounted?).
root@ubuntu:/# sudo chroot /media/disk/
chroot: cannot change root directory to /media/disk/: No such file or directory
root@ubuntu:/# sudo chroot /media/disk-2/
chroot: cannot change root directory to /media/disk-2/: No such file or directory
root@ubuntu:/#