For future referance, the simplest trick is to just keep ejecting and re-inserting the disk. Sometimes only a very small change in the alignment of the disk in the drive is enough to make it readable or unreadable. You could also make a point of putting the disk in at slightly different angles, but usually just ejecting and re-inserting it is sufficient to make it readable. Also, like you were doing, it is a good idea to try the disk in more than one computer because sometimes the slight differences of the alignment of the heads in different drives will also make it readable in one drive but not in another.