I did not fix the problem in conventional Linux way, I used Windows. I have decided to work with what is familiar to me.
I installed Windows 10 really quick on a new 50Gb partition to use some of its tools for hot fix (Windows takes ~>20Gb). I ran error checking with fixing and it seems ok.
Going back to Manjaro it says that I am not authorised to mount this hard drive, for which I do the following:
lsblk - I search for my drive to mount (sdc1).
sudo mount /dev/sdc1 /mnt/ - then I mount it. It takes a while, but after that I can access my drive.
In any case, I might need to reformat the drive to ext4, if you just unplug the USB from the laptop, instead of “unmounting” first, the drive can’t be mounted back again.