I just loaded Mint 18 on my main PC over the Mint 17 boot. Unfortunately, it has caused a boot problem with GRUB. However, I can go to BIOS and boot up into Windows, and that partition seems unaffected, so I think I'll be OK. I just need to try to fix my boot loader, or if all else fails, I can just switch my BIOS options and load into Windows.
Annoying, but when I first booted into the GRUB error screen, I was very concerned I had just screwed up my Windows partition, too.
I think Gparted might can fix that.
Or this other thing I used to use. Some kind of universal boot/partition loader fixer thing.
Yeah, I'm drawing a blank. Nowadays, I just put whatever on whatever disk, haven't dual-booted in 5 years.
I'd put mint on an 8gb microSD and call it good.
BCD-something comes to mind as for the fixer program.