Here are some key points (out of order) from your Boot Repair output:
/dev/sda1 * 2,048 669,571,071 669,569,024 7 NTFS / exFAT / HPFS
In other words, Windows is on /dev/sda1
. There are other lines that confirm this, including the next ones, which are important for other reasons....
sda1: __________________________________________________________________________
File system: ntfs
Boot sector type: Grub2 (v1.99-2.00)
In other words, GRUB is installed in the boot sector of /dev/sda1
, your Windows partition. This is highly irregular at best, and really is the source of the problem; but to complete the loop, as it were....
menuentry 'Windows 8 (loader) (on /dev/sda1)' --class windows --class os $menuentry_id_option 'osprober-chain-B6F60302F602C29B' {
insmod part_msdos
insmod ntfs
set root='hd0,msdos1'
This fragment of your GRUB configuration confirms that GRUB is trying to launch the boot loader on /dev/sda1
(hd0,msdos1
in GRUB-speak) for Windows. Ordinarily this would work, but because that partition holds GRUB instead of the Windows boot loader, you end up in the endless loop you describe.
Chances are this problem arose because you told Ubuntu to install GRUB to /dev/sda1
at some point. This was a mistake, but if you did this through the Ubuntu installer, it's arguably a bug that the installer accepted this location, which it could (and arguably should) detect as an invalid option and refuse to accept, or at least warn you of the danger. If you think this is what happened, you should probably file a bug report to encourage changes that would prevent this sort of thing from happening to others (or to yourself!) in the future.
As to fixing it, I'm afraid that you'll have to use Windows tools for this job. I'm not an expert on repairing Windows in this way, so unless somebody else chimes in with details, you may do well to post to a Windows forum. The key is that you need to restore the Partition Boot Record (PBR) for the first partition. Ideally, you do not want to touch the Master Boot Record (MBR); however, there's a good chance that your Windows repair will replace the MBR as well as the PBR. If this happens, you'll need to run Boot Repair again to re-install GRUB to the MBR.