I recently installed Ubuntu 12.04 alongside my Debian Wheezy and Win 7 partitions. I decided that I didn't like Ubuntu, and wanted to remove it. I backed up the entire drive, then booted into Debian and deleted the Ubuntu / and home partitions with gparted. I then ran update-grub, which seemed to work fine; it found the remaining bootable partitions, wrote grub.cfg, and closed without error.

However, on reboot I got the dreaded grub rescue> prompt. At this point, I took the easy way out and reinstalled Ubuntu from the DVD. This worked; grub is back to normal now.

But I still want to remove Ubuntu. How can I do this without messing up grub?

I don't have Ubuntu (or Debian for that matter) around the house, but I'll bet update-grub only builds the cfg. Have a look for grub-install to actually write the MBR.

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Thanks; I actually figured that out and tried to update the post, but the site was down. I don't why it hadn't occurred to me that update-grub didn't actually write to the MBR. I deleted the partition again, ran update-grub again just to be on the safe side, then did grub-install /dev/sda, and now it boots with the updated grub.