Debian installation issue

[Linux for blind general discussion]

I haven't set up a dual boot since the XP days, but I remember Windows
Boot Loader not playing nice with non-Windows OSes and have no reason
to think Microsoft would be any less hostile to dual booting today
than they were in 2005, and as far as I know, the boot record of the
first hard drive has always been the recommended installation location
for Grub.

Someone with more recent dual booting experience could probably offer
better advice, but short of disconnecting the Windows drive, my
suggestion would be trying to install Grub to the boot record of sda.