I hate trying to use hard drive magnets. They are typically of irregular shape, difficult to center and difficult to balance properly. The rare earth magnets are relatively inexpensive; not free, but not very costly either.
IMO, you need some fairly strong magnets for the best performance. I've been using some very strong rare earth magnets. You can vary the magnet pulling force by adjusting the distance between the magnets and the stir bar. The magnetic force is inversely proportional to the square of the separation distance. This means that a small change in the gap will result in a substantial change in the magnetic force. So, get some bad a$$ strong magnets and adjust the gap as needed.