Thankyou. The solution found in that persons post doesn’t work for me. I forgot about the ntfsfix command. I think I used it on the same drive a couple of years ago when I was having a similar issue, and the referenced post reminded me of that.
However, what does allow me to mount the drive is doing it manually through the terminal, as suggested in that post here, using the command sudo mount -t ntfs /dev/sdc1 /mnt/.
This doesn’t fix the issue though. Because if I plug the HDD out and back in again, and try to select it in Dolphin to mount it, I still get the same error as before. I tried turning off auto-mount options before plugging the HDD in and out again as well, but that didn’t seem to do anything; I still get the same error.
So for now I can at least access the files on the HDD, but it’s inconvenient to have to manually mount it in the terminal every time.