I have been dual-booting Windows XP SP1 and Fedora Core 4 on separate hard drives (henceforth known as 160 and 120, respectively). GRUB boot loader was installed to the MBR of 160 (the Windows drive). Last night the 120 (Linux) started clicking, and today it failed.

I've used Fedora on 120 twice. Last time it was dual-booting with Windows XP SP1 as well, on a hard drive henceforth known as 80. When it started clicking, I thought 80 had failed, but apparently it was actually 120 that was failing, because if I put 80 or 160 in another computer, they don't click. But they cannot boot because they both give a GRUB Hard Disk Error during boot and can't get any further than that.

I don't know much about how GRUB actually works, since I'm just starting to get into Linux. But it seems maybe once it's installed to the MBR, the drive with the MBR needs the Linux drive present and working in order to boot successfully? The two Windows drives seem fine other than the GRUB Hard Disk Error.

I'm no expert with grub, but most bootloaders store their configuration file on the linux drive under /boot. Generally I make a separate /boot partition on whichever hard drive I'm installing to the MBR. That way if I nuke the linux partition, the bootloader still works fine.

Now the good thing is that with grub I think you can recover. Again, I'm no expert, but there's a way to access a grub "prompt" that will let you boot the partitions from a command line.

Alternatively, you can put in your Windows bootdisk and fixmbr to restore the original windows bootloader.

You can also boot Linux using a Live CD, then drop to the command line and manually edit any files as necessary. That said though, 120 needs to be replaced. If you are able to mount it and access it, do as much of a backup as you can with any important data, but from your description I would not be optimistic that 120 will come back, and even if it does, that's probably only a temporary condition. Not to be pessimistic but once a drive crashes like that it cannot be fully trusted any longer

That's what I've done, both Windows drives are working fine now.

en.opensuse.org/SDB:Uninstalling_the_Boot_Manager_GRUB_from_the_MBR#Using_Windows_XP
(sorry about the link, but it's useful and relevant, not an advertisement)

So I assumed. Fortunately all I had on it was a fresh FC4 installation, no personal files or anything.