what is meant by /dev/<primary_disk> in Installing a system.7?

so far so good with this but am having trouble getting installed system to boot. unsure of this bit:

chroot ./rootfs grub-install /dev/<primary_disk>

what is primary_disk?

this is my partition layout:

root@hellen-ubunntu:~# fdisk -ls

Disk /dev/sda: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 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: 0x8cdd4106

   Device Boot Start End Blocks Id System
/dev/sda1 1 25190 202334278 7 HPFS/NTFS
/dev/sda2 * 25190 25203 102400 7 HPFS/NTFS
/dev/sda3 25203 121602 774322177 5 Extended
/dev/sda5 25203 49691 196701773 7 HPFS/NTFS
/dev/sda6 49691 75188 204800000 83 Linux
/dev/sda7 75188 76208 8192000 82 Linux swap / Solaris
/dev/sda8 76208 77483 10240000 83 Linux
/dev/sda9 77483 77610 1024000 82 Linux swap / Solaris
/dev/sda10 77610 80160 20480000 83 Linux
/dev/sda11 80160 80670 4096000 82 Linux swap / Solaris

/dev/sda8 is where ive built lfs and /dev/sda10 is where i've installed it