I've read several dozen very detailed "repair grub bootloader using Linux live CD."
Either didn't (fully) apply or didn't work.
Problem suddenly appeared - I did not just install a new OS or anything. Vista 64 bit & storage partitions are on one HDD.
Mint 18.1 cinnamon 64 bit is on an identical 2nd HDD. For months, Grub2 menu appeared at boot.
Mint was at top of grub menu, then some other Mint options (Advanced, etc) & Vista was last.
It defaulted to booting Mint.
"Something" happened & grub menu stopped displaying. Though Mint is still at top of menu, it now defaults booting Vista (which works).
When the Vista HDD is disconnected, the Grub menu appears as always & boots Mint in a few seconds.
But if Vista HDD is REconnected, grub menu doesn't show & boots to Vista.
I did the "reinstall grub from live CD" thing & then update-grub. A lot of work due to having to mount several partitions & a Mint bug for Nvidia GPUs w/o proprietary drivers.
It obviously replaced some files - the modified date changed on some but by no means, all the grub files.
** Question: seems odd that during grub install, it showed "installing for i386-PC platform...", when I have 64 bit Mint & Vista. Is that the way it should be? I see the Grub folder named "i386" & all files in it have modified dates match when I installed Grub. Lots of other grub folders / files weren't updated, so I don't know if it completed.
Now, I can get Mint booted, maybe I should run install-grub again from inside Mint, then update-grub??
I found a way to make the grub menu appear. Hold both the Shift & Esc keys during post screen. That shows a DOS looking boot menu. All it shows is the 2 identical HDD's & DVD drive.
Selecting the lower HDD actually brings up the normal Grub menu (still showing Vista), but works at booting what used to be the default Mint.
** All I need is details how to get Grub boot menu to display all the time & default to Mint.
I looked in the file /etc/default/grub. Saw a post on it https://askubuntu.com/questions/148095/how-do-i-set-the-grub-timeout-and-the-grub-default-boot-entry#148097
GRUB_DEFAULT=0 #GRUB_HIDDEN_TIMEOUT=0 GRUB_HIDDEN_TIMEOUT_QUIET=true GRUB_TIMEOUT=10 GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian` GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" GRUB_CMDLINE_LINUX=""
The '#' in front of (my) hidden timeout (commenting the line) may explain the menu not showing, but since Mint (normal mode) is the grub menu's TOP entry, it should default to loading Mint, not Vista. Unless another file overrides these settings.
FYI: the entry above for GRUB_DISTRIB. is identical to mine.