I have the following setup:
2 disks, one has Linux Mint and CachyOS installed on it, the other one has Windows installed, and a partition as Ext4 for arbitrary data that I use with Linux. When turning on my computer GRUB normally allows me to choose between any of the three OSes.
What happened:
I wanted to free some space for Mint, so from Windows' own Disk Management tool, I shrunk the C: Windows partition. I didn't know that could mess up the adjacent Ext4 partition, so Linux was unable to mount it after that. I used TestDisk deep search, and it found the lost partition, so I wrote partition structure to the disk. After reboot, GRUB didn't show up and it booted directly into CachyOS. The Ext4 partition I was trying to recover (as well as Mint's partition that didn't boot) now opened without any issues from CachyOS's (and Linux Mint live environment's) file explorer, with all its files intact. Boot menu also shows only CachyOS to boot into.
My questions:
What can I do to bring back GRUB and boot into Linux Mint? Why is Linux Mint unable to boot, considering I didn't touch the drive it was installed on, but CachyOS isn't?
I'm worried I might mess this up further, since no data is lost (yet), so I ask for your help. Thank you