I have my OS (Manjaro) installed on a small SSD and I have a separate larger SSD attached (both are EXT4) to store my personal files, however I can't modify the larger SSD through Dolphin, the option to create folders is greyed out and I can't copy anything to it. I am guessing it's a permissions issue, how do I do this?

I believe this happens because the external storage with the ext4 file system has write permission granted only to the root. When you create an ext4 partition in KDE Partition Manager (I'm looking at version 22.04.3), it lets you choose between giving permissions to root or to everyone. But now that you have already created that partition, you need to change that permission to be able to use it normally. I think there are (at least) two ways for doing this, the first one is through GUI. You first need to launch dolphin as root in the /media folder, then open the properties' dialog for that device, and then modify the permissions from the permissions tab. The second method is through terminal, and I'm not very familiar with that myself, but here is a link to a fully explained article: https://itsfoss.com/set-write-permissio ... ntu-linux/

Thanks. I used GParted to format it the first time, now I re-formatted it with GNOME Disk Utility and that gave me permission to modify it, no idea why that worked.