Here is a quick example of grub commands that might just work, explanations and caveats below.
Most likely for post-2010 installs.
Most likely for pre-2005 installs.
For the 2005-2010 period, your guess is as good as mine, but if you use the command for the wrong version, you only get a harmless syntax error on the first command.
Choosing the right root
At startup, grub will probe for your devices and assign numbers to them. All devices that are partitioned (hard disks and flash drives) will also have numbers assigned. The format is
grub2, partition indexes changed, so the two examples above have the same effect despite looking to use different roots.
Your first device (
hd0) is whichever device grub just loaded from. After that, you can usually assume that all the internal devices will come before your external devices. They will most likely be in the form of
hd and a number.
After the comma is the partition index. Hard disks and thumb drives will almost always be partitioned, so you must choose the right (and most likely only) partition. CD-ROMs are usually not partitioned.
More documentation: http://www.gnu.org/software/grub/manual/html_node/Device-syntax.html
When choosing your root partition, you can use the Tab key to probe for device names and partition indexes. Just open parenthesis and start pressing Tab to see the list.