I have been cooking a roast beef for 7 hours on low but it doesn't look like it will be cooked for another two hours. Can I put it on high or will it go tough?
There should be no problem cooking for an extra hour or two on 'high'. The rule of thumb which I have learned is one hour on "high" = two hours on "low". Generally, you should always start with an hour on "high" in order to get the food out of the danger area for bacterial growth as soon as possible, after which you can reduce the heat to "low".