I installed multiple Linux distros to test them out and learn more about each to see which I like best. I installed two (Ubuntu 18.04 LTS and Mint 19) on a 256 GB SSD (sdc) and four (KDE Neon, Fedora 29, Manjaro Deepin and CentOS 7, in that order) on a 1 TB HDD (sde).
I can see both distros when I boot into the SSD (sdc), and I could see the first three on the HDD (sde) in it's grub menu, but after installing CentOS, booting it and logging in, it locked up on me. I rebooted, but found that I can only see KDE Neon in grub when I boot to the HDD (sde).
I've read a lot about using grub-install, update-grub and update-initramfs, but I'm not sure which option (or combination) would work here. I was able to successfully get rid of a swap partion on the SSD (sdc) using the instrucitons I found here, but I don't know how much of that would help in this situation. I would like to see all six distros in the grub menu of either sdc or sde, if possible, as well as Windows 10 (on sda). Windows boots fine from its separate SDD, so having it in grub isn't a necessity.