The HDD (LDM) is used for generic files, like pictures, music files etc.
The SSD (NTFS) is used for games.
The second SSD (ext4) is the system disk.
In a fresh Manjaro install, the LDM and NTFS disks have to be manually mounted.
In the HDD I have full access to my OneDrive folder.
Questions:
If I go to Settings > Removable Storage > Removable Devices, and check the boxes “on login” for the LDM and NTFS partitions, they won’t be automatically mounted after I reboot. What is that setting for?
When I use Disks (Gnome), I can successfully set the partitions to mount at startup, but the OneDrive folder in the HDD becomes just a broken shortcut.
Never used those options myself, but the proper way to mount partitions on boot is using fstab. You can see all devices that will be mounted in /etc/fstab. However, fstab is sensitve, therefore try not to mess with unless you know what you are doing, a faulty fstab can result in an unbootable OS (if this happens you have to fix the fstab file somehow, e.g. using a live system).
I’d suggest using the KDE partition manager. Just right-click the desired partition and select “edit mount point” → you maybe want to mount it by UUID, configure a path (default is /run/media/<username>/<partitionname>) and make sure to add the “nofail” option to prevent an unbootable OS. For more info see fstab - ArchWiki
I don’t use OneDrive myself but usually Windows is “faking” some directories, or better said just providing a shortcut, as in this case. You have to find the real location where windows does keep the OneDrive files, maybe this is helpful: https://superuser.com/questions/1473136/real-location-of-onedrive-folder-in-windows-10
Then you can for example just add the real location to the dolphin places list (shortcuts in the left pane of dolphin) for quick access.