[ubuntu] problem with installing GRUB

I had replaced xubuntu with ubuntu and after the instalation I wanted to install grub using live cd. And this happens:

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0xc3ffc3ff

Device Boot Start End Blocks Id System
/dev/sda1 * 2048 206847 102400 7 HPFS/NTFS/exFAT
/dev/sda2 206848 411523071 205658112 7 HPFS/NTFS/exFAT
/dev/sda3 411525118 976773119 282624001 5 Extended
Partition 3 does not start on physical sector boundary.
/dev/sda5 411525120 472571903 30523392 83 Linux
/dev/sda6 472573952 480962559 4194304 82 Linux swap / Solaris
/dev/sda7 480964608 976773119 247904256 7 HPFS/NTFS/exFAT

Disk /dev/sdb: 15.5 GB, 15479597056 bytes
32 heads, 63 sectors/track, 14996 cylinders, total 30233588 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x5bba1f16

Device Boot Start End Blocks Id System
/dev/sdb1 * 63 30231935 15115936+ c W95 FAT32 (LBA)
root@ubuntu:/home/ubuntu# grub-install /dev/sda5
/usr/sbin/grub-probe: error: cannot find a device for /boot/grub (is /dev mounted?).