Background:
It is a dual boot machine with F18 installed on /dev/sda3. In order to boot F18, I have created (during F18 installation) an MBR on a USB stick. So now when I insert the USB stick, my laptop boots F18 system (otherwise it boots Win7). MBR on HDD boots only Win7 and I do not want to touch or modify it. I have a good reason for that, which I do not want to discuss here.

Problem description:
Yesterday I lost my USB stick. Without USB stick I can't boot F18...

Question:
How can I (re)create MBR on another USB stick (without having to reinstall F18) that will point to /dev/sda3 as the boot partition?

If you really do mean MBR then I'd think a few things. One would be that it is the 512Kb data (or more) on the first part of the disk. It has a few parts. One part being the partition table data.

I'd use some tool maybe a live cd like partition magic or such to get the current data off your hard drive before you start. I know Norton had some partition info tools that I used to use. Maybe Ranish can be used from ultimate boot cd or rescue cd.

To put a generic one might use a command like

ms-sys --partition /dev/sdax (be sure x is your usb drive boot order double check)


Some live cd's would also offer a way to boot to the hard drive and you could use it. Some live cd's might be installed briefly to the usb to use it's loader and mbr.

Pretty sure you could use syslinux or even windows to put a loader on the usb that might allow you to load linux.

Usually I'd think grub install would do this but I am not that good at grub/2

Grub4dos or Grub4nt would or should also work. Maybe even plop loader or other loader or multiboot loaders.

I assume you may have to put grub or some loader some place or chained off to some place if it isn't already on the hard drive or new usb.

I'm sure others would have some ideas.

Simply boot any liveCD/USB and chroot into the installed F18 system. "grub-install /dev/sdb" should do the job - providing you hit the correct USB drive node. The cfg should be o.k. as-is seeing as you want to run the hard disk system once booted, but in need you can run grub2-mkconfig - Fedora doco here.

N.B. - Fedora is a bit different to (for example) Ubuntu in its grub2 setup. Be careful using docs off the web that are (even unknowingly) heavily Debian-centric.

This may also be worth a look at.

http://www.supergrubdisk.org/

--- edited ---

It took me some trial and error, but eventually I successfully recreated MBR on the USB stick.

The exact procedure is:
1) Use gparted to create NTFS partition on the USB stick (and make it bootable)
2) Start F18 Live DVD
3)

Thanks for the update.