What does grub-install do?

[Dennis Cao]

Hi buddies,

I want to install GRUB2 on my USB disk, so that I can boot my computer
if my system crashes.

I read manual of GRUB2 and tried to understand what grub-install did,
especially the '--root-directory' parameter. The manual says:
     "... ....  If you want GRUB to use images under a directory other
than the root directory, you need to specify the option
--root-directory .... ...."

I think grub-install first creates and installs 'boot.img' to MBR. And
it hardcodes the location of core.img inside boot.img. So, does
'--root-directory' tells grub-install to use core.img in the specific
directory?

Another question. I use the following command to try to install grub
to my usb disk.
          grub-install --root-directory=/boot  /dev/hdb
But when I boot my computer from USB disk,  my computer just stops
after printing 'GRUB' on the left-top conner of the screen :(

Can anybody explain the details and usage of grub-install command?
What does it do? Which images are used and where are they copied to on
the hard or usb disk?

 Thanks in advence:)

-- 
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曹贵林 ( Dennis Cao )