Fix a grub after windows update

Question

I recently set up a dual-boot on my Windows computer. Windows 10 and Ubuntu 18.04 using grub. After a Windows update I could no longer boot the grub although I can boot straight into windows.

I've been trying to follow various guides to fix this. I select "try Ubuntu" when I boot from a thumb drive.

When I try to mount the drive Ubuntu is on (/dev/sde1) I get

Mount: /mnt/chrootdir: /dev/sde1 already mounted on /cdrom

When I try to umount I get

Umount: /cdrom: target is busy.

When I try sudo mount -t vfat /dev/sde1 /mnt/chrootdir I get

/dev/sde1 already mounted or mount point busy.

I'm assuming that it was the Windows Update that did this, but it could be something else.

Does anyone have any ideas how I can fix the grub/Ubuntu?
Or is there a better way to set up a dual-boot to avoid problems like this?

Solution

I ended up just using the boot repair instructions instead. That still didn't fix the problem but it led me back into the UEFI BIOS where I went into "Secure Boot" and I changed the option from "Windows" to "Other OS". Finally, this fixed the problem for me.

Comments

[sdexp]

In case anyone has the same problem, when I looked up the list of partitions Ubuntu looked like it was /dev/sde1 but when I did the boot-repair it looked like it was something else.

[u123]

WARNING! After running those commands I can no longer access BIOS and it just boots windows directly.