Here we go starting fresh.
In your case, Sylvia and Serena may be in the opposite positions.GNU GRUB version 2.02~beta2.36ubuntu3.14
Then (I have numbered the lines)
1. Linux Mint 18.3 Cinnamon 64-bit
2. Advanced Options Linux Mint 18.3 Cinnamon 64-bit
3. Memory test (memtest86+)
4. Memory test (memtest86+)serial console 115220
5. Linux Mint 18.1 Serena (18.1) (on /dev/sdxy)
6. Advanced Options Linux Mint 18.1 Serena (18.1) (on /dev/sdxy)
(by Chris - for /dev/sdxy substitute your partition and number)
sudo xed /etc/default/grub
# grub-mkconfig -o /boot/grub/grub.cfg
Yes, as you saw in #9 above, there are three (3) methods I can think of easily, but each of the first two (1. & 2.) involves an element of risk:That can't be changed?
If you manage to get a handle on being able to boot from a bootable stick, then we can revisit the options.Viewers and Helpers should note that while Dick has a Unetbootin stick with Linux Mint 18.3 'Sylvia' Cinnamon and Persistence on it, burned - he has so far been unable to sidestep his BIOS to get it to boot from the stick ahead of the HDD or SSD.
So no stick ability, no fresh install, nor Timeshift restore, if Serena is blown away.
No comment(You just might need a breather from Doc for a while!)