LG DR4810 DVD recorder says disk is full when it's not.

I am trying to record various bits onto a DVD-R which is supposedly 120mins, but after recording an 8 minute clip the DVD recorder says the disk is full. It is not finalized, but I cannot add anything else to it. It has obviously done something to the disk, as my laptop says it is full too, but I can see from the marks left on the disk that only a tiny bit of it has been used. Please help!