Cannot boot into Seven after installing Build 9926

After installing Windows 10 build 9926 tonight in a dual boot set up with Seven, I can no longer access Seven at all. When I select to boot into Seven, all I get is a black screen.

When I force a restart I get a pointless disk check that finds no new errors (a bad 4kb sector shows up, but that is old).

I can sometimes get to the Windows Seven startup repair/restore screen, but running either of them does not seem to fix the problem.

I can usually get into Ten (like I am now), but even then I have issues... like my mouse will not work about half of the time.

I really need to get my Seven back... and no, a re-installation of Seven is not in the picture.

How do I fix this mess?

Edit: It looks like my boot sector has been moved from C:/ Seven to Y:/ Ten... and I don't know where that F:/ partition came from!

The disk checks are due to the fast start option. Disable that and they will go away:

Fast Startup - Turn On or Off in Windows 10 - Windows 10 Forums

Setup Windows 7 as the default boot option and see if that resolves your black screen.

Thanks for the advice. Both suggestions worked great! Now I can figure out a good way to remove this Ten installation, fix the boot sector, and re-install it again.

Thanks!

In Win7, you should use the Backup feature to create and burn a Win7 Repair CD. You can then boot from that, run Startup Repair (you might have to run it three times), and that will restore the Win7-based boot loader files.