It’s most likely corruption, if the files can be seen, but not written to.
Or, possibly, if you’re not safely removing the drive, but just pulling the cable, you could be locking the drive to being written to. At least, that’s what I’ve heard. Try connecting, then safely removing, to see if the drive gets unlocked for writing to. If it doesn’t you’re stuck with reformatting the drive and starting over. However, you lose the data, if you have to do that.
