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I'm not sure what you mean by a link to the OS. If you mean that you don't have a grub menu entry to the original Red Hat installation, your best bet would be to boot up in Fedora, and then mount the Red Hat boot partition. The /boot/grub/menu.lst file will contain the section for booting into the Red Hat installation. Simply copy that portion from your old menu.lst file (RHEL) and past it into the current menu.lst file (FC). When you boot, grub accesses the menu.lst file, and that is what is used to produce the menu. By copying those lines to the new menu.lst file, you will be adding another menu item.
If you use lilo instead, then you need to run the "lilo" command after editing /etc/lilo.conf. Red Hat and Fedora use grub however. Unless you started using Lilo instead, you can forget about this.
Fine, grub accesses the menu.lst file from the last distro where you ran grub-install. Use that menu.lst file to make changes. Also, print it out and make a backup on a floppy or cdrom or pen drive. Also, print out and backup the output of "fdisk -l". That can help you recover from accidently wiping out your partition table during a future install.
Using grub, you don't need to keep running grub-install if grub is already on the MBR. That is because the information needed to boot is contained in the menu.lst text file on the boot partition. This is unlike Lilo which stores the starting and ending location of the kernel on the device. If you update the kernel, you must run "/sbin/lilo" again. With grub, you just need to edit the file.
You can also use the grub shell to locate the RHEL partition that contains the kernel and vmlinuz files. That and the root partition are enough for you to boot up with, so it is possible to boot up from a live distro's grub. So if you had reinstalled windows and lost grub on the MBR, you could still boot up into Linux, edit the menu.lst file if needed. ( I this example that wouldn't be necessary ), and run grub-install to put grub back on the MBR. Just so that you understand, only run grub-install on the same distro. That way you won't run into dueling distro syndrome. (dds!!!) One thing you may find useful is to have the /home directory on its own partition and share it among the distro's. You will want to use a different user home directory in most cases, but you could use the same username, but have a modified user home directory name. When installing a new distro and adding a regular user, add a suffix indicating the distro. So johndoe's home page on rhel could be /home/johndoe-rhel and on Fedora Core /home/johndoe-fc. Good Luck!
The last distro that you installed. The grub-install program was run using the menu.lst from the Fedora Core's /boot/grub/menu.lst file. That is why there wasn't a menu entry for RHEL. Hopefully you created a separate partition for the /boot directory. That would A) make it easy to backup because it is small. B) Make it easier to mount from a live distro or one of the other distro's; if you use ext3 and it isn't part of an lvm volume.