In my laptop, I have a 128GB SSD (/dev/sdb) and a 1TB HDD (/dev/sda). Before today, I had Ubuntu 18.04 installed on a 30GB partition (sdb5) on the SSD alongside Windows 10 on a 90GB partition (sdb3).
Now I chose to install a minimal Lubuntu on the HDD for times when I just need to browse the web or do simple activities with minimal battery drain. I created an 7.5GB partition for / (sda4) as well as a 512MB swap partition (sda3) and a 5MB BIOS partition (sda2). The last one I made without really knowing what I was doing because otherwise I was getting a partition table requires a separate partition for boot loader code error. I left the biggest, sda1 partition as my data partition for all systems.
After installation finished and the PC rebooted, I was greeted by my regular Ubuntu GRUB, located on the SSD from the previous Ubuntu install. Normally I'd be happy with that result - except this GRUB doesn't seem to see the Lubuntu partition. I checked GRUB Customizer and it doesn't show up there either.
My question is: How do I make the Ubuntu GRUB from the SSD see the Lubuntu on the HDD?
You could try adding "GRUB_DISABLE_OS_PROBER=false" to your /etc/default/grub file and then do a "sudo update-grub".
Add "GRUB_DISABLE_OS_PROBER=false" without quotes if it's not there already and then:
It should scan for other partitions in other drives and add the entry to your grub.cfg
Apologies to everyone who answered/commented... Before you had time to, I managed to try another fix that worked. Basically something that fixes most boot-related problems (I swear this software is magical), meaning I just ran
boot-repairand voilà, problem solved.